You might think of your teeth as tools, like built-in knives and forks, but if they are mere tools, why do they feel pain and wouldn’t it be better if they could just【C1】_____under any condition? In spite of our【C2】__discomfort, it turns out there’s a good reason our teeth are so sensitive. Tooth pain is a【C3】_____mechanism that ensures when a tooth is being damaged we’ll notice and do something about it.

If we eat something too hot or too cold, or if the tooth is worn down enough where the tissue【C4】_____is exposed, all of those things cause pain, and then the pain causes the person not to use that tooth to try to protect it a little bit more. So it’s really a protective mechanism more than anything else. If teeth didn’t feel pain, we might 【C5】__to use them in situations that damage them, and for humans, damaging 【C6】__teeth is a problem because, unlike crocodiles, we can’t【C7】_____ them.

Teeth have three layers, only one of which—the innermost layer of the tooth—can hurt, as that layer of the tooth【C8】_____both blood vessels and nerves. Pain is the only feeling to which the nerves in that layer respond. Whereas people with tooth sensitivity may complain, for example, of tooth pain【C9】__by heat or cold, the nerves in the inner layer don’t sense temperature. Rather, they feel pain, which may be【C10】_____with, say, drinking something very cold.

A) adult I) emotional

B) associated J) implies

C) chew K) mammal

D) contains L) replace

E) continue M) swallow

F) defense N) triggered

G) dental O) underneath

H) downward

【C2】