题目内容：

Winter begins in the north on December 22nd. People and animals have been doing what they always do to prepare for the colder months. Squirrels(松鼠), for example,  have been busy gathering nuts from trees. Well, scientists have been busy gathering information about what the squirrels do with the food they collect.

They examined differences between red squirrels and gray squirrels in the American state of Indiana. The scientists wanted to show how these differences could affect the growth of black walnut(黑胡桃)trees. The black walnuts is the nut of choice for both kinds of squirrels. The black walnut tree is also a central part of some hardwood forests.

Rob Swihart of Purdue University did the study with Jake Goheen, a former Purdue student now at the University of New Mexico. The two researches estimate(估计)that several times as many walnuts grow when gathered by gray squirrels as compared to red squirrels. Gray squirrels and red squirrels do not store nuts and seeds in the same way. Gray squirrels bury nuts one at a time in a number of places. But they seldom remember where they buried every nut. So some nuts remain in the ground. Conditions are right for them to develop and grow the following spring. Red squirrels, however, store large groups of nuts above ground. Professor Swihart calls "death traps for seeds".

Gray squirrels are native to Indiana. But Professor Swihart says their numbers began to decrease as more forests were cut for agriculture. Red squirrels began to spread through the state during the past century.

The researchers say red squirrels are native to forests that stay green all year, unlike walnut trees. They say the cleaning of forest land for agriculture has helped red squirrels invade Indiana. Jake Goheen calls them a sign of an environmental problem more than a cause.

1.The study done by Rob Swihart and Jake Goheen is to ____ .

A. Find out how squirrels collect walnuts

B. Learn squirrels' influence on black walnut trees

C. Do something to get rid of squirrels

D. Save the forests in the American state of Indiana

2.The difference between gray squirrels and red squirrels lies in ____.

A. The way they gather the walnut    B. The time they have winter sleep

C. The place they have winter sleep    D. The place they store the walnuts

3.What could be inferred from this text?

A. Agricultural in Indiana has been well developed

B. Gray squirrels will be replaced by red squirrels in Indiana.

C. The spread of red squirrels will do harm to walnuts trees in Indiana

D. The government will take some measures to protect black walnut trees.