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 clocks. 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] 四个选项A项的原形insect“昆虫”，B项的原形bird“鸟”，C项fish“鱼”，D项的原形snake“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...biological clock...”为主旨句。后面的句子都是作者举出的生物钟调节生物行为的例子。