题目内容：

    Bees may soon need to add protein shakes to their diet. Pollen (花粉) normally provides their protein (蛋白质). But rising levels of carbon dioxide (CO2) in the air have reduced the protein in pollen. That’s what a new study finds.

Scientists compared recently collected flowers from Canada goldenrod. Then they compared pollen in them to pollen in goldenrod reserved at a museum in Washington, DC. They found today's plants have less protein. Protein levels in pollen from the older flowers were 18 percent. Today's goldenrod pollen has only about 12 percent protein. That's about one-third less than 172 years ago. Over that same period, CO2 levels have gone up by more than one-third. Back more than 170 years ago, they were about 280 parts per million in air. Today they are about 398 ppm.

To test whether CO2 changes had played a role in the falling protein levels, the researchers did some tests. They grew goldenrod for two years at CO2 levels of up to 500 ppm. More CO2 in the air led to lower levels of protein in the flowers’ pollen.

Joan Edwards, one of the researchers, says that this drop in pollen protein could affect a bee’s diet and health. It would be like eating junk food, tasting good but offering less nutrition (营养).

Honeybees need to eat lots of protein. They use it to feed their young. They also need it to keep their immune systems healthy, says Cédric Alaux, a bee biologist at the French agricultural research agency.

Bee populations have been decreasing worldwide in recent years. A drop in their food quality might play a role, Edwards says. And that drop might affect crops too. “The health of the bee population is not just for the flowers and the bees and biodiversity.” She worries it also could play a role in “human health and well-being.”

1.The second paragraph is mainly developed by __________.

A. giving examples B. following the time order

C. explaining reasons D. providing numbers

2.According to the text, Joan Edwards might agree that ____________.

A. the drop of protein in pollen has a good influence on bees

B. today’s flowers can provide bees with enough nutrition

C. the drop of protein in pollen leads to a smaller population of bees

D. the smaller population of bees has nothing to do with the human race

3.What is the main idea of the text?

A. Today’s flowers contain more protein.

B. Protein levels in pollen connect with CO2 levels.

C. Drop of protein in pollen affects bees.

D. Bee population is becoming smaller recently.