Every
living thing has what scientists call a biological clock that controls behavior.
The biological clock tells {{U}} {{U}} 1 {{/U}} {{/U}}when to
form flowers and when the flowers should open. It tells {{U}} {{U}}
2 {{/U}} {{/U}}when to leave the protective cocoons and fly away, and it
tells animals and human beings when to eat, sleep and wake.
Events outside the plant and animal {{U}} {{U}} 3 {{/U}}
{{/U}}the actions of some biological docks. Scientists recently found, for
example, that a tiny animal changes the color of its fur {{U}} {{U}}
4 {{/U}} {{/U}}the number of hours of daylight. In the short {{U}}
{{U}} 5 {{/U}} {{/U}}of winter, its fur becomes white. The fur
becomes gray brown in color in the longer hours of daylight in summer.
Inner signals control other biological clocks. German scientists found
that some kind of internal clock seems to order birds to begin their long
migration {{U}} {{U}} 6 {{/U}} {{/U}}twice each year. Birds
{{U}} {{U}} 7 {{/U}} {{/U}}flying become restless when it is
time for the trip, {{U}} {{U}} 8 {{/U}} {{/U}}they become calm
again when the time of the flight has ended. Scientists say
they are beginning to learn which {{U}} {{U}} 9 {{/U}} {{/U}}of
the brain contain biological clocks. An American researcher, Martin Moorhead,
said a small group of cells near the front of the brain {{U}} {{U}}
10 {{/U}} {{/U}}to control the timing of some of our actions. These
{{U}} {{U}} 11 {{/U}} {{/U}}tell a person when to {{U}}
{{U}} 12 {{/U}} {{/U}}, when to sleep and when to seek food.
Scientists say there probably are other biological clock cells that control
other body activities. Dr. Moorhead is studying {{U}}
{{U}} 13 {{/U}} {{/U}}our biological clocks affect the way we do our
work. For example, most of us have great difficulty if we must often change to
different work hours. {{U}} {{U}} 14 {{/U}}
{{/U}}can take many days for a human body to accept the major change in work
hours. Dr. Moorhead said industrial officials should have a better understanding
of biological clocks and how they affect workers. He said {{U}} {{U}}
15 {{/U}} {{/U}}understanding could cut sickness and accidents at work
and would help increase a factory's production.
【答案解析】[解析] 4个选项A项insects“昆虫”；B项birds“鸟”；C项fish“鱼”；D项snakes“蛇”。很显然只有昆虫会形成茧。故本题选A。本段第一句“Every living thing has what scientists call a biological clock that controls behavior.”为主旨句。后面句子都是作者举出的生物钟调节生物行为的例子。