Question 18 is based on the passage you have just heard.

听力原文：

    Wild carrots probably evolved with the other flowering plants, about 360 million years ago. Like apples, carrots are native to Central Asia. That’s why horses, which come from Central Asia, like both apples and carrots so much. 

    （16）With wild carrots, the roots are white, small and skinny, so you’d have to pick a lot of wild carrots to get enough to eat. Doctors used carrot seeds and roots as medicine, on the theory that foods that taste bad must be good for you.

    Around 800 AD, people in Central Asia managed to develop a new kind of carrot—a purple carrot—that attracted more interest from international traders.（17）Then in the late 1500s, food scientists in the Netherlands cultivated large, straight, sweet, red carrots like the ones we eat today. But people still mostly fed carrots to horses, donkeys and pigs, and didn’t eat them themselves. In the 1600s, people in China used carrots as medicine, but they also ate carrots boiled in soup. The red color was popular for Chinese New Year celebrations.（18）But carrots got their biggest boost during the two world wars, when food shortages forced people to eat them, and government told everyone how healthy carrots were. Today, cooler countries grow most of the world’s carrots. Machines do most of the planting and picking, and carrots are easy to store and ship, so carrots are cheap almost everywhere.

18. Why did people turn to carrots for food during the two world wars?

解析：B。本题目为细节题。问题为：在两次世界大战期间，为什么人们开始把胡萝卜当成食物？出题点位置在转折处。根据原文“But carrots got their biggest boost during the two world wars, when food shortages forced people to eat them, and government told everyone how healthy carrots were.”，可见两次世界大战极大地促进了人们食用胡萝卜，那时候食物短缺，并且政府告诉人们胡萝卜是很健康的。故正确答案为B）Food shortages.（食物短缺）。