The Sun is the hub of a huge rotating system consisting of nine planets, their satellites, andnumeroussmall bodies, including asteroids, comets, and meteoroids. An estimated 99.85 percent of themassof oursolarsystem is contained within the Sun, while the planetscollectivelymake up most of theremaining0.15 percent. The planets, in order of their distance from the Sun, areMercury,Venus, Earth, Mars, Jupiter,Saturn, Uranus, Neptune, andPluto. Under the control of the Sun'sgravitationalforce, eachplanetmaintains anellipticalorbitand all of them travel in the same direction.

The planets in oursolarsystem fall into two groups: theterrestrial(Earth-like) planets (Mercury,Venus, Earth, and Mars) and the Jovian (Jupiter-like) planets (Jupiter,Saturn, Uranus, and Neptune). Plutois not included in eithercategory, because its great distance from Earth and its small size make thisplanet's true nature a mystery. The mostobviousdifferencebetween theterrestrialand the Jovian planets is their size. The largestterrestrialplanet, Earth has adiameteronly one quarter as great as thediameterof the smallest Jovianplanet, Neptune, and itsmassis only one seventeenth as great. Hence, the Jovian planets are often called giants. Also, because of theirrelativelocations, the four Jovian planets are known as the outer planets, while theterrestrialplanets are known as theinnerplanets. There appears to be a correlation between the positions of these planets and their sizes.

Other dimensionsalongwhich the two groups differmarkedlyaredensityandcomposition. The densities of theterrestrialplanetsaverageabout 5 times thedensityof water, whereas the Jovian planets have densities thataverageonly 1.5 times thedensityof water. One of the outer planets,Saturn, has adensityof only 0.7 that of water, which means thatSaturnwouldfloatin water. Variations in thecompositionof the planets are largelyresponsiblefor thedensitydifferences. The substances that make up both groups of planets are divided into three groups—gases, rocks, and ices—based on theirmeltingpoints. Theterrestrialplanets are mostly rocks:denserockyandmetallicmaterial, withminoramounts of gases. The Jovian planets, on the other hand,containa large percentage of the gaseshydrogenandhelium, with varying amounts of ices: mostly water,ammonia, andmethaneices.

The Jovian planets have very thick atmospheres consisting of varying amounts ofhydrogen,helium,methane, andammonia. Bycomparison, theterrestrialplanets havemeageratmospheres at best. Aplanet'sabilitytoretainanatmospheredepends on itstemperatureandmass. Simplystated, a gasmoleculecan "evaporate" from aplanetif it reaches a speed known as theescapevelocity. For Earth, thisvelocityis 11 kilometers persecond. Anymaterial, including a rocket, mustreachthis speed before it can leave Earth and go intospace. The Jovian planets, because of their greater masses and thus higher surface gravities, have higherescapevelocities (21-60 kilometers persecond) than theterrestrialplanets. Consequently, it is more difficult for gases to "evaporate" from them. Also, because the molecularmotionof a gas depends ontemperature, at the low temperatures of the Jovian planetseventhe lightest gases areunlikelytoacquirethe speed needed toescape. On the other hand, acomparativelywarm body with a small surfacegravity, like Earth's moon, is unable to holdeventhe heaviest gas and thus lacks anatmosphere. Theslightlylargerterrestrialplanets Earth,Venus, and Marsretainsome heavy gases likecarbondioxide, buteventheir atmospheres make up only an infinitesimally smallportionof their totalmass.

The orderly nature of oursolarsystem leads most astronomers toconcludethat the planets formed atessentiallythe same time and from the samematerialas the Sun. It is hypothesized that theprimordialcloud of dust and gas from which all the planets are thought to have condensed had acompositionsomewhatsimilarto that of Jupiter. However, unlike Jupiter, theterrestrialplanets today are nearly void of light gases and ices. The explanation may be that theterrestrialplanets were once much larger and richer in these materials buteventuallylost them because of these bodies'relativecloseness to the Sun, which meant that their temperatures wererelativelyhigh.

Questions:
question_stem: According to the passage, each of the following statements comparing terrestrial planets with Jovian planets is true EXCEPT:
choices: tag: A
text: Terrestrial planets are closer to the Sun than Jovian planets.

tag: B
text: Terrestrial planets have smaller diameters than Jovian planets.

tag: C
text: Terrestrial planets have smaller masses than Jovian planets.

tag: D
text: Terrestrial planets travel in a different direction than Jovian planets do.
answer_key: D
explanation: D EXCEPT 题，排除法，问整个文章，所以应该关注各段的开头。第二段提到了分成两类，第三四两段叙述区别；A 的 closer 做关键词定位至第三段倒数第二句，正确，不选；B 的 diameter 做关键词定位至第三段第二句，正确，不选；C 的 smaller masses 做关键词定位至第三段第一句的 size，正确，不选；D 与原文第一段末句相反，错，选。

question_stem: The word markedly in the passage is closest in meaning to
choices: tag: A
text: Essentially

tag: B
text: Typically

tag: C
text: Consistently

tag: D
text: noticeably
answer_key: D
explanation: D markedly: 显著的。A 是必要地，本质上地；B 是典型地；C 是一贯地，一致地；D 是显著的。所以答案是 D 的 noticeably。

question_stem: Paragraph 3 mentions which of the following as a reason why terrestrial planets are dense?
choices: tag: A
text: They are made up of three groups of substances.

tag: B
text: They are composed mainly of rocky and metallic materials.

tag: C
text: They contain more ice than Jovian planets.

tag: D
text: They contain relatively small amounts of water.
answer_key: B
explanation: B 以 dense 做关键词定位至第一句，但这句只是单纯说类地行星比类木行星密度大，没给出原因，正确答案在第三句的 density difference，说组成物质的不同导致两者密度的差异，所以答案是 B，主要由金属和岩石构成。A 不是原因 C 说反 D 没说。

question_stem: Paragraph 3 supports each of the following statements about Saturn EXCEPT:
choices: tag: A
text: It is less dense than any of the terrestrial planets.

tag: B
text: It contains no rocky material.

tag: C
text: It contains ices.

tag: D
text: It contains a large percentage of gases.
answer_key: B
explanation: B EXCEPT 题，排除法，A 的 dense 做关键词定位至第一和第二句，说类地行星的密度大致是水的五倍，土星是水的0.7倍，所以 A 说土星比任何类地行星密度都小正确，不选；B 的 rocky material 做关键词定位至倒数两句，说类地行星主要由石质物质组成，类木行星主要由气态物质组成，没说土星所属的类木行星没有石质物质，错，选；同时证明 C 和 D 正确，不选。

question_stem: The word meager in the passage is closest in meaning to
choices: tag: A
text: rich

tag: B
text: thin

tag: C
text: unique

tag: D
text: complex
answer_key: B
explanation: B meager: 稀少的，稀薄的。A 是丰富的；B 是稀薄的；C 是独特的；D 是复杂的。所以答案 B 正确。

question_stem: According to paragraph 4, which of the following statements is true of both Jovian and terrestrial planets?
choices: tag: A
text: The thicker the atmosphere, the smaller the planet’s mass

tag: B
text: The more varied the gases in the atmosphere, the higher the temperature

tag: C
text: The higher the surface gravity, the higher the escape velocity

tag: D
text: The less the atmosphere contributes to the total mass, the lower the temperature
answer_key: C
explanation: C 因为同时问到两种星星，所以不能单独找任何一种，接着刚才的题往下看，说行星保持大气的能力取决于其温度和体积，下面分述了体积越大，温度越低，气体越难以逃离，也就是逃逸速度越大，大气层越厚。所以 A 和 D 说反，B 没说，答案是 C。正选不会的话可以用排除法。

question_stem: According to paragraph 4, what is a major reason that Jovian planets have much thicker atmospheres than terrestrial planets do?
choices: tag: A
text: Jovian planets have lower surface gravities

tag: B
text: Jovian planets have lower temperatures

tag: C
text: Jovian planets have lower escape velocities

tag: D
text: Jovian planets’ gas molecules have higher average speeds
answer_key: B
explanation: B 接上题，类木行星体积大，温度低，引力大，逃逸速度大，所以 A 和 C 说反；B 正确。原文倒数第三句说气体分子的运动速度取决于温度，温度越低，分子的运动速度越小，所以 D 和 B 事实上是反的，所以 D 也说反了。

question_stem: Paragraph 4 supports which of the following statements about the ability of planets to retain gases?
choices: tag: A
text: More-massive planets are less able to retain gases than less-massive ones.

tag: B
text: Planets are more likely to retain heavy gases than light gases.

tag: C
text: Jovian planets are unlikely to retain the lightest gases.

tag: D
text: Only terrestrial planets have been able to retain carbon dioxide.
answer_key: B
explanation: B 接上题，类木行星体积大，温度低，引力大，逃逸速度大，更容易有厚的大气层，所以 A 和 C 说反了；而 D 明显不对，所以正确答案是 B。

question_stem: In calling the cloud of gas and dust from which the Sun and all the planets are thought to have condensed "primordial," the author means that the cloud was
choices: tag: A
text: immense in size

tag: B
text: composed of similar particles

tag: C
text: present at the very beginning of our solar system's formation

tag: D
text: created from a great variety of different materials
answer_key: C
explanation: C 修辞目的题，修辞点所在句子只是说了一个事实，所以往前看，前一句说太阳系形成的有序性使得天文学家相信太阳系和太阳本身是同时形成的。既然是有序性，后文肯定要开始说这个顺序了，所以答案是 C，太阳系形成最初的时候，而且整个段一直在强调时间概念，所以 C 正确。

question_stem: The word eventually in the passage is closest in meaning to
choices: tag: A
text: over time

tag: B
text: long ago

tag: C
text: simply

tag: D
text: certainly
answer_key: A
explanation: A eventually: 最后，最终。A 是随着时间的过去；B 很久以前；C 是简单地；D 是当然。所以 A 的 over time 正确。

question_stem: According to paragraph 5, what is a possible explanation for the lack of light gases and ices on terrestrial planets?
choices: tag: A
text: The location of terrestrial planets caused them to lose some of the materials they once contained.

tag: B
text: Terrestrial planets were formed much later than Jovian planets.

tag: C
text: The composition of terrestrial planets was different from that of Jupiter.

tag: D
text: Terrestrial planets were formed out of different material than the Sun was.
answer_key: A
explanation: A   以 lack of gases and ices 做关键词定位至倒数第二句，这句话只是单纯说现象，最后一句才是解释，而且这句话刚才已经讲过，说由于距离太阳近，类地行星最终失去了这些气态物质，所以答案是 A。其余答案均与本段首句说反。