The first thing to understand about eating spicy food is that it really isn’t a matter of taste. Capsaicin (辣椒素), the active chemical in capsicum (辣椒) plants that are a key ingredient in anything you’d think of as “spicy”, evolved as an irritant to stop mammals from chewing and destroying plant seeds. It acts on the nervous system directly through receptors in the tongue, throat and skin — no taste buds required — and, in theory, tells our bodies that the thing we’ve just ingested is something to get rid of as soon as possible. The obvious question, then, is: why do some of us like the sensation so much?

Firstly, some people have different variants of the TRPV1 gene, which change how easily it switches on and turns off after activation (interestingly, the gene is present but insensitive to capsaicin in all birds and one species of tree-shrew <树鼩>, letting them eat spicy seeds without harm). But the receptors also become less sensitive over time, so it’s entirely possible to develop a relative immunity to moderately spicy foods — or deliberately cultivate one.

As for why we like it, that might be because of the sensation it causes. “Research is still ongoing into how human brains learn what is and isn’t safe, including which foods are safe for consumption,” says Browne. “Recent work emphasises prediction, context and controllability. The idea is that when you eat something spicy, you get an initial alarm-like ‘heat’ signal. With repeated exposure the body’s outer response grows less sensitive, and the brain learns the cue is safe and under control. That shift in prediction and certainty is a large part of why the experience becomes tolerable — and then rewarding.” The more spice you eat, the theory goes, the less intense the first painful part becomes, and the more relief you get afterwards.

1.1.According to the passage, what is the primary function of capsaicin in plants?

A Protect themselves.

B Enhance flavor.

C Attract birds.

D Promote growth.

解析：选A。A细节理解。根据第一段内容，辣椒素进化作为刺激物来阻止哺乳动物咀嚼和破坏植物种子，因此主要作用是“阻止哺乳动物”（stop mammals from chewing and destroying plant seeds），从而保护自己。选项B、C、D均为植物可能的功能，但并非辣椒素的主要作用。故选A。

2.2. The underlined word “it” in Para. 2 refers to the ________.

A receptor

B gene variant

C capsaicin

D nervous system

解析：选A。A词意指代题。定位到划线单词所在句，“it”出现在关系代词“which”引导的定语从句中，而“which”指代的是前面的“different variants of the TRPV1 gene”（TRPV1基因的不同变体），结合科学可知，“it”指代的不是基因变体（variants ）本身，而是由该基因编码的受体（receptor）。故选A。

3.3.What can be inferred about someone who regularly eats spicy food?

A Their TRPV1 gene has changed.

B Their pain receptors become less sensitive.

C They have completely lost the feeling of heat.

D They need less spice to feel any effect.

解析：选B。B推理判断题。从第二段中的“受体随着时间变得不敏感 ( the receptors also become less sensitive over time)”和第三段的“身体的直接反应变得不那么敏感(With repeated exposure the body’s outer response grows less sensitive)”可以推断，常吃辣的人其疼痛受体敏感性会降低。A项（基因改变）原文未提及；C项（完全失去热感）过于绝对；D项（需要更少辣度）是不太可能的结果。故选B。

4.4.What is the main idea of the last paragraph?

A The dangers of eating spicy food.

B Different reactions to capsaicin.

C How to relieve the burn of capsaicin.

D The chemical properties of capsaicin.

解析：选C。C段落大意题。最后一段主要回答了“如果你觉得太辣该怎么办”的问题，并解释了喝水和牛奶的不同效果，核心是“如何缓解辣椒素带来的灼烧感”。A项（危险）未强调；B项（不同反应）太宽泛；D项（化学属性）是解释方法的原因，不是段落主旨。故选C。