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.
13
A、In any case
B、In this case
C、In a sense
D、In no sense
【正确答案】：B
【题目解析】：荷尔蒙这种化学物质能控制身体的某些机能；荷尔蒙为鸟类向北飞行做了准备。第五段第二句话说，the factors are more difficult to explain.荷尔蒙对迁徙的作用也只是实验证明，in any case,in no sense均表示"无论如何，不管怎样"，不合题意。in a sense指“在某种意义上”；in this case表示“假使是这样的话”，符合文意。答案为B。
