It is thought that thefacialpits orpitorgans on the head of some snakes arespecializedinfrared(heat) receptors. When a rattlesnake strikes, the direction of thestrikeseems to be guided by theinfraredradiationfrom itsprey. A rattlesnake strikes only at warm-bloodedprey, and when thepreyis dead and at roomtemperature, the snake will notstrike. However, a blindfolded snake strikes correctly at a dead rat that is pulledacrossthe cage, provided the rat is warmer than the surroundings. Blindfolded, the snake cannot be guided byvision; nor is it guided by thesenseof smell, for it willstrikecorrectlyevenat a moving, cloth-wrapped electricbulb. Thepitorgans are evidently involved in sensing the location of warm objects. All snakes that havepitorgans feed preferentially on warm-bloodedprey, and this further supports the view that these organs areinfraredsensors. In the rattlesnake thepitorgans are located, one on each side, between the nostril and the eye; they are connected to many nerves, and this in itself suggests asensoryrole for the organ.

The sensitivity of thefacialpithas been examined by recording theactivityin the nerveleadingfrom the organ. Avarietyof stimuli, such assound,vibration, or light ofmoderateintensity (with theinfraredpart of thespectrumfiltered out), has nodetectableeffect on theactivityin the nerve. However, if objects of atemperaturedifferent from the surroundings are brought into the receptivefieldaround the head, there is astrikingchange in nerveactivity,regardlessof thetemperatureof the interveningair.

How is theinfraredradiationsensed? Thepitis covered by a thintransparentmembrane, and it has been suggested that a rise intemperaturein thepitbehind themembranecould cause anexpansionof the gas with aconsequentdeformation of themembrane. This in turn could be sensed by asuitablereceptor. Thishypothesisis highlyimprobable, for a cut in themembranethat opens thepitto the outsideaircauses no loss in responsiveness, a result that isincompatiblewith thehypothesisthat apressurechange is sensed.

We are left with two other possibilities to consider: Either the effect is photochemical, which means that theinfraredradiationis absorbed by aspecificcompound,analogousto the light-sensitive pigments in the eye, or thepitorgans aresensitiveto theslighttemperaturerise caused wheninfraredradiationreaches it. Theinfraredradiationemitted from amammalianbody has low quantum energy, which makes any photochemical effect on apigmentextremelyunlikely. Pureinfraredradiationcan be produced by alaser, and experiments with suchradiationof known wavelength provide strongevidencethat themodeofreceptionin thefacialpitorgan isentirelythermal.

Can thepitorgans be used forperceptionof theinfraredsourcein the way our two eyes are used for stereoscopicvision? This seems likely, not only from observations of the precision with which a snake canstrike, but also from studies of its brainactivity. Wheninfraredradiationfalls on thefacialpitorgan, electricactivitycan be recorded from the optic tectum, the part of the brain with which the optic nerve is connected. This in itself is suggestive; although the nerves from thepitorgan are completelyseparatefrom the optic nerves, the same part of the brain seems tohandlevisualinformation, which is known to be stereoscopic, andinfraredinformation. The optic tectum has left and right parts, located on either side of the brain. Each part receives input from each eye via the optic chiasma, the point at which the optic nerves from the left and right eye meet. Many of the neurons in the tectumrespondtostimulationof thepitorgan on theoppositeside of the head. This is reminiscent of the way information from the eyes is handled; the crossover of the optic nerve in the optic chiasma isessentialfor stereoscopicvisionandinterpretationof distance. Information from the twopitorgans is apparently coordinated and interpreted in asimilarway, a conclusion in agreement with recorded changes in the neuralactivityin the tectum when theinfraredsourceis in apositionto irradiate both pits at once. Itthereforeappears that thefacialpits indeed provide stereoscopicperceptionandsubstantiallyaid in the precision of estimating the location ofprey.

Questions:
question_stem: According to paragraph 1, a rattlesnake will only strike at prey when
choices: tag: A
text: the snake is able to see the prey

tag: B
text: the prey emits chemical odors

tag: C
text: the prey is moving

tag: D
text: the prey is warmer than its surroundings
answer_key: D
explanation: 定位题干信息 rattlesnake…strike at prey 到第三、四句话：A rattlesnake strikes only at warm-blooded prey，when the prey is dead and at room temperature, the snake will not strike。 However, a blindfolded snake strikes correctly at a dead rat that is pulled across the cage, provided the rat is warmer than the surroundings.得出结论就是 rattlesnake 仅会攻击温血动物，且高于环境温度，故而选 D。

question_stem: Why does the author mention a “cloth-wrapped electric bulb” in the passage?
choices: tag: A
text: To illustrate the size of prey usually attacked by snakes

tag: B
text: To prove that blindfolded snakes will strike more often at a dead rat than an electric bulb

tag: C
text: To eliminate the possibility that snakes locate prey by using a sense of smell

tag: D
text: To explain how scientists were able to trick blindfolded snakes
answer_key: C
explanation: 定位题干所在信息句：Blindfolded, the snake cannot be guided by vision; nor is it guided by the sense of smell, for it will strike correctly even at a moving, cloth-wrapped electric bulb.大概意思是 Blindfolded 蛇不受嗅觉和视觉的引导，因为它能正确地击中一个移动的、布包裹的电灯泡。定位信息在句子中为原因部分，提到原因是为了说明结果，结果正好是前半句内容。也就是作者提到蒙住眼睛的蛇可以攻击到包裹着的灯泡，既不是靠 vision 也不是靠 smell，故而选 C。

question_stem: Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
choices: tag: A
text: However, if objects in the surrounding area experience a temperature change, nerve activity causes a change in temperature in the receptive field around the snake’s head.

tag: B
text: When objects that differ in temperature from the surroundings enter the receptive field around the head, the result is a dramatic change in nerve activity.

tag: C
text: Nerve activity changes when new objects are introduced into the area around a snake’s head, regardless of how their temperature compares to that of the surrounding air.

tag: D
text: The temperature of the air in the receptive field around the snake’s head has an effect on the nerve activity of the snake if the objects in the field are at a different temperature.
answer_key: B
explanation: 该句子核心考点为 if 条件关系，原句大概意思为：如果将温度与周围环境不同的物体带到头部周围的感受区，神经活动都会发生显著变化。B 符合这种条件关系，当与环境不同温度的物品进入到头部感受区域，神经活动会显著变化。A if 条件描述与原文不符合，nerve activity causes a change in temperature 这点无中生有；C regardless of how their temperature compares to that of the surrounding air 这点错了，与原文物品温度与周围不同不相符合；D if 条件与原句不符合，且结果部分也不符合；

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

tag: B
text: eventual

tag: C
text: premature

tag: D
text: visible
answer_key: A
explanation: consequent：随之发生的，作为结果的；符合同义的只有 A。resultant：因而发生的；注意 B 选项是先后关系，不一定有因果关系。

question_stem: Which of the following can be inferred from the discussion about infrared sensation and pit organs in paragraph 3?
choices: tag: A
text: Rattlesnakes have special receptors that detect pressure changes in the pit organ membrane.

tag: B
text: Damaged pit organ membranes do not prevent a rattlesnake from detecting a warm animal nearby.

tag: C
text: The experiments on pit organ membranes were conducted in a chamber with air at outside temperatures.

tag: D
text: The idea that pit organs help snakes to locate prey by radiation has been disproved.
answer_key: B
explanation: 题干问从关于 infrared sensation and pit organs 的讨论中可以推出什么？整段考察。A 错，未提到 special receptors that detect pressure changes；B 正确，最后一句中提到 a cut in the membrane that opens the pit to the outside air causes no loss in responsiveness，膜上的一个切口将凹坑向外界空气打开，不会造成反应性的损失，即受损的膜也可以收到周边的温度信号；C 错，文章未提到实验环境情况相关的信息，无中生有；D 错，原文未否定 pit organs 可以用来定位探测猎物这一观点。

question_stem: According to paragraph 4, which of the following tends to exclude a photochemical explanation for the sensory function of snake pit organs?
choices: tag: A
text: Only lasers can produce the exact wavelengths of radiation that have an effect on pit organs.

tag: B
text: Light-sensitive pigments are located in the eyes but not in the pit organs of snakes.

tag: C
text: Compounds sensitive to infrared radiation have not yet been identified in snake pit organs.

tag: D
text: The infrared radiation from a snake’s prey has too little energy to affect the snake’s pigments.
answer_key: D
explanation: 题干问那一选项排除了 a photochemical explanation？结合文本中的一二两句，第一句提出了两种可能解释，一种是 photochemical，infrared radiation is absorbed by a specific compound，类似于 light-sensitive pigments，另一种是 pit organs are sensitive to the slight temperature rise caused when infrared radiation reaches it；第二句话反驳了第一个 photochemical，哺乳类动物中释放出的 infrared radiation has low quantum energy，不太可能有任何 photochemical effect。故而答案符合的是 D，猎物释放的热辐射中所含的能量太少，不太可能 affect snake’s pigments。

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

tag: B
text: undoubtedly

tag: C
text: frequently

tag: D
text: significantly
answer_key: D
explanation: substantially：实质上，大量地，充分地；符合同义转述的为 D significantly,大量地。

question_stem: According to paragraph 5, all of the following support the idea that a snake uses its pit organs for stereoscopic perception EXCEPT the fact that
choices: tag: A
text: stimulation of a pit organ has an effect on the tectum on the opposite side of the brain

tag: B
text: the same part of the brain that deals with stereoscopic vision is activated by infrared stimulation of the pit organs

tag: C
text: the nerves of the right eye and the right pit organ are completely separate from the nerves of the left eye and the left pit organ

tag: D
text: a snake is able to strike its prey with precision
answer_key: C
explanation: 题干问以下哪一个事实不能支持观点：snake uses its pit organs for stereoscopic perception。A 选项对应到本段第7句话，Many of the neurons in the tectum respond to stimulation of the pit organ on the opposite side of the head，正确。B 选项对应到本段第4句，although the nerves from the pit organ are completely separate from the optic nerves, the same part of the brain seems to handle visual information, which is known to be stereoscopic，and infrared information，正确。D 选项对应到本段第二句：This seems likely, not only from observations of the precision with which a snake can strike，正确。C 选项错误，与本段倒数第二句中的信息描述相反：Information from the two pit organs is apparently coordinated and interpreted in a similar way。