Lightningis abrilliantflashof light produced by an electricaldischargefrom a storm cloud. The electricaldischargetakes place when theattractivetensionbetween aregionof negatively charged particles and aregionof positively charged particles becomes so great that the charged particles suddenlyrushtogether. The coming together of the oppositely charged particles neutralizes the electricaltensionand releases atremendousamountof energy, which we see aslightning. The separation of positively and negatively charged particles takes place during thedevelopmentof the storm cloud.

The separation of charged particles that forms in a storm cloud has a sandwich-like structure. Concentrations of positively charged particles develop at the top and bottom of the cloud, but the middleregionbecomes negatively charged. Recent measurements made in thefieldtogether withlaboratorysimulationsofferapromisingexplanation of how this structure of charged particles forms. What happens is that small (millimeter-to centimeter-size) pellets of ice form in the cold upper regions of the cloud. When these ice pellets fall, some of themstrikemuch smaller ice crystals in the center of the cloud. Thetemperatureat the center of the cloud is about -15℃ orlower. At such temperatures, thecollisionbetween the ice pellets and the ice crystals causes electrical charges toshiftso that the ice pelletsacquireanegativecharge and the ice crystals become positively charged. Thenupdraftwind currents carry the light, positively charged ice crystals up to the top of the cloud. The heavier negatively charged ice pellets are left toconcentratein the center. Thisprocessexplains why the top of the cloud becomes positively charged, while the center becomes negatively charged. The negatively chargedregionis large: several hundred meters thick and several kilometers indiameter. Below this large, cold, negatively chargedregion, the cloud is warmer than -15℃, and at these temperatures, collisions between ice crystals and falling ice pellets produce positively charged ice pellets that thenpopulatea smallregionat the base of the cloud.

Mostlightningtakes placewithina cloud when the charge separationwithinthe cloud collapses. However, as the storm cloud develops, the groundbeneaththe cloud becomes positively charged andlightningcan take place in the form of an electricaldischargebetween thenegativecharge of the cloud and the positively charged ground. Lightningthat strikes the ground is the most likely to bedestructive, soeventhough it represents only 20 percent of alllightning, it has received a lot ofscientificattention.

Using high-speedphotography, scientists havedeterminedthat there are two steps to theoccurrenceoflightningfrom a cloud to the ground. First, achannel, orpath, is formed that connects the cloud and the ground. Then a strongcurrentof electrons follows thatpathfrom the cloud to the ground, and it is thatcurrentthat illuminates thechannelas thelightningwe see.

Theformationof thechannelis initiated when electronssurgefrom the cloud base toward the ground. When astreamof these negatively charged electrons comeswithin100 meters of the ground it is met by astreamof positively charged particles that comes up from the ground. When the negatively and positively charged streams meet, a completechannelconnecting the cloud and the ground is formed. Thechannelis only a few centimeters indiameter, but that is wide enough for electrons to follow thechannelto the ground in thevisibleform of aflashoflightning. Thestreamofpositiveparticles that meets thesurgeof electrons from the cloud often arises from a tallpointedstructure such as a metal flagpole or a tower. That is why thesubsequentlightningthat follows the completedchanneloften strikes a tall structure.

Once achannelhas been formed, it is usually used by severallightningdischarges, each of them consisting of astreamof electrons from the cloud meeting astreamofpositiveparticlesalongthe establishedpath. Sometimes, however, astreamof electrons following an establishedchannelis met by apositivestreammaking a newpathup from the ground. The result is a forkedlightningthat strikes the ground in two places.

Questions:
question_stem: According to paragraph 1, all of the following take place in the development of a flash of lightening EXCEPT
choices: tag: A
text: great tension between two oppositely charged regions

tag: B
text: an increase in negatively charged particles over positively charged particles

tag: C
text: oppositely charged particles coming together

tag: D
text: the release of electrical energy in the form of visible light
answer_key: B
explanation: B EXCEPT 题，排除法，A 的 tension 和做关键词定位至第二句，正确，不选；B 的 negatively 和 positively charged particles 同样定位至第二句，但原文没有比较，所以 B 没说，选；C 的 come together 和 D 的 release 做关键词定位都至倒数第二句，都正确，都不选。

question_stem: The word “tremendous” in the passage is closest in meaning to
choices: tag: A
text: distinct

tag: B
text: growing

tag: C
text: huge

tag: D
text: immediate
answer_key: C
explanation: C tremendous: 大量的，巨大的。A 是明显的，独特的；B 增加的；C 是巨大的；D 是立即的。所以正确答案是 huge。

question_stem: According to paragraph2, what causes ice crystal to become positively charged?
choices: tag: A
text: Collisions with ice pellets

tag: B
text: Collisions with negatively charged ice crystals at the base of the cloud

tag: C
text: Becoming concentrated in the central region of the cloud

tag: D
text: Forming at a temperature greater than -15℃
answer_key: A
explanation: A, 以 ice crystal 和 positively charged 做关键词定位至第七句，说在这样的温度下，ice pellet 和 ice crystal 的碰撞转移了电荷，使得 pellet 带负电，crystal 带正电，所以答案应该是 A 碰撞；B 虽然也说到碰撞，但不是和负电碰撞；D 的温度不是带正电的决定条件。

question_stem: The word “acquire” in the passage is closest in meaning to
choices: tag: A
text: reject

tag: B
text: obtain

tag: C
text: need

tag: D
text: produce
answer_key: B
explanation: B acquire: 获得。A 是拒绝；B 是获得；C 是需要；D 是生产。所以答案 obtain 正确。

question_stem: According to paragraph2, why are positively charged ice pellets produced in the lower part of the cloud?
choices: tag: A
text: Collisions between ice crystals and ice pellets increase in number in the lower part of the cloud.

tag: B
text: The lower part of the cloud is smaller than the region above it.

tag: C
text: More ice pellets than ice crystals reach the lower part of the cloud.

tag: D
text: Temperature in the lower part of the cloud are warmer than -15℃.
answer_key: D
explanation: D 以 positively charged ice pellets 做关键词定位至最后一句，说在这个很大的，带负电的区域之下，碰撞产生了 positively charged pellets，所以答案是 D。A 的 increase in number，B 的 smaller 和 C 的比较原文都没说。

question_stem: According to paragraph2, the middle region of a cloud becomes negatively charged due to all of the following EXCEPT
choices: tag: A
text: a shift of electrical charged between ice pellets and ice crystals

tag: B
text: negatively charged ice pellets that remain in the middle

tag: C
text: a temperature of -15℃ or less

tag: D
text: the development of a positive charge at the base of the cloud
answer_key: D
explanation: D  EXCEPT 题，排除法，A 的 ice pellets and ice crystals 做关键词定位至第七句，正确，不选；B 的 in the middle 做关键词定位至倒数第四句，正确，不选；C 的数字做关键词定位至第六句，正确，不选；D 的 the base of the cloud 做关键词定位都至最后一句，但倒数第三句已经说过 this process explains，也就是说原因到倒数第三句就给完了，D 不是原因，错，选。

question_stem: It can be inferred from paragraph 2 that part of the reason that the top of a storm cloud becomes positively charged is that
choices: tag: A
text: the top of the cloud is warmer than the middle of the cloud

tag: B
text: the middle of the cloud is already occupied by positively charged particles

tag: C
text: the negatively charged ice pellets are too heavy to be carried by the updrafts that move ice crystals

tag: D
text: collisions between ice pellets in the top of the cloud produce mainly positively charged particles
answer_key: C
explanation: 以 top of the storm cloud 做关键词定位至倒数第五句：

Then updraft wind currents carry the light, positively charged ice crystals up to the top of the cloud.

说向上的气流将轻的，带正电的粒子带到云层上部，这自然是上部云层带正电的原因，反过来说也就是带负电的粒子很重，所以没有上来，所以答案是 C。原文只是说了中部和下部的温度，上部温度没有信息，所以 A 错；由倒数第四句可知中部是负电，B 说反了；D 原文也没说。

question_stem: The author remarks that “Lightning that strikes the ground is the most likely to be destructive” in order to explain why
choices: tag: A
text: this form of lightning has been investigated so much

tag: B
text: this form of lightning is not as common as lightning within a cloud

tag: C
text: scientific understanding of this form of lightning is important

tag: D
text: the buildup of positive charge on the ground beneath a storm cloud can have serious consequences
answer_key: A
explanation: A  段落最后一句话，后半部分说“It has received a lot of scientific attention.”这种形式的闪电受到了很多科学关注。只有选项一符合，其余选项都不符合。

question_stem: The word “illuminates” in the passage is closet in meaning to
choices: tag: A
text: opens

tag: B
text: completes

tag: C
text: lights

tag: D
text: electrifies
answer_key: C
explanation: C illuminate 发光，照射，所以 C 的 light 是正确答案。原句说一股强烈的电子流沿通路从云层到地面，并且怎么样了那个通路，成为我们所看到的闪电，如果不照亮怎么看到闪电呢？所以 C 正确；A 打开 B 完成 D 使带电都不能解释看见，所以都不对.

question_stem: According to paragraph5, which of the following is true of the stream of charged particles from the ground?
choices: tag: A
text: It prevents streams of electrons from the cloud from striking the ground.

tag: B
text: It completes a channel that connects the storm cloud with the ground.

tag: C
text: It produces a stream of electrons from the cloud.

tag: D
text: It widens the path made by the initial stream of electrons from the cloud.
answer_key: B
explanation: B 以 stream of charged particles from the ground 做关键词定位至第二句，说来自地面的带负电的电子流会在地面以上100米左右碰上来自云层的带正电的电子流，然后又说当二者碰在一起的时候就形成了通路，所以 B 形成通路是正确的。A 的 prevent，C 的 produce 和 D 的 widen 原文都没说.

question_stem: Which of the following claims about lightning strikes can be inferred from paragraph 5?
choices: tag: A
text: During a lightning strike the diameter of the channel the electrons follow is considerably enlarged beyond a few centimeters.

tag: B
text: A building is unlikely to be hit by lightning unless it is at least 100 meters tall.

tag: C
text: A building is hit by a lightning strike because the building itself has first determined the path the lightening then takes to it.

tag: D
text: The light of a lightning strike first appears at the point where the streams of negative and positive particles meet.
answer_key: C
explanation: C 因此题关键词比较难找，所以用排除法。A 的 diameter of the channel 和 centimeter 做关键词定位至第四句，但原文没有信息能推出 widen，所以 A 错；B 的数字做关键词定位至第二句，说100米内正负电荷就会碰到，所以 B 说反；C 的 path 做关键词定位至最后一句的 channel，说正电荷通常升起自地面上的一个高点，比如旗杆，然后闪电由通路到达地面，也就是说旗杆等高的建筑物是这个 channel 的一部分，所以 C 说 building 决定了 path 是对的；D 的 first appear 原文没有相关信息 .

question_stem: The word “initiated” is closet in meaning to
choices: tag: A
text: started

tag: B
text: intensified

tag: C
text: finished

tag: D
text: expected
answer_key: A
explanation: A，initiate: 开始。A 是开始，B 是加强；C 是完成；D 是盼望，预期。所以正确答案是 A。