CLOZE
Fill in each of the 15 blanks in the passage with the most likely answer.
Migration, in biology, is the movement of animals to a place that offers better living conditions. Many birds, fishes, insects, and mammals regularly migrate to （1） unfavorable changes in weather or food supply．
Migrations take place on land, in water, or in the air. Some animals migrate only short （2） . Other migrations cover thousands of miles or kilometers．
Migration enables many animals to （3） favorable weather and abundant food supplies in areas with changing （4） . In some parts of the world, for example, food is plentiful in summer but becomes （5） during the cold winter months. Many animals that live in these regions migrate to （6） climates in the fall. They return in the spring when the weather warms up．
Many migrations are （7） reproduction. Numerous animals migrate to breeding areas so that their young have the best chance for （8）. Migratory birds breed in their summer homes, （9） the food supply is most abundant．
Many animals begin their migrations after unfavorable environmental conditions （10） . But among other species, the factors that trigger (引发) migrations are more difficult to explain. Experiments show that changes in day length （11） the migrations of many species of birds. In spring, the （12） hours of daylight trigger the release of certain hormones in the bodies of the birds. Hormones are chemical substances that regulate many body functions. （13） , the hormones prepare the birds for the northward trip．
Besides using environmental （14） , many seasonal migrators probably have an inborn "calendar" that tells them when to migrate. Some birds show seasonal migratory behavior even when kept （15） constant conditions in a laboratory. An inborn timing mechanism may trigger the migration of salmon and other animals that migrate at different stages of their life.
第5题
A、scarce
B、frozen
C、poisonous
D、tasteless
【正确答案】：A
【题目解析】：由“but”可知句子前后是转折关系，前半句说的是食物充足，那么后半句应为食物短缺或不足。 scarce意为“缺乏的，不足的”，A选项符合题意。句意：例如，在世界上的一些地区，夏天食物充足，但在寒冷的冬季，食物却变得很缺乏。
