CAREFUL READING
Read the following passages carefully. Decide on the best answer and choose the corresponding letter.

     On a January day in 1975, Ken and Catalina Brugger wandered through an ancient forest in Mexico on a high mountain slope eighty miles west of Mexico City. The air was damp and cool. The sky was cloudy, so little light reached through the trees. As the Bruggers walked along, they realized they were hearing a quiet, constant noise. It was like rain falling on the fir tress. But there was no rain. They looked around for the source of the sound. Suddenly, sunlight broke through the clouds and lit up the forest. The Bruggers gasped in delight. All around them, the trees shimmered with the beating of brilliant orange and black wings. The Bruggers were surrounded by millions of monarch butterflies, resting in their winter home.
     The Brugger’s discovery was important in the world of butterfly study. Butterfly lovers knew that, late every summer, monarchs migrate from Canada into Mexico. More than 300 million of the fragile creatures make the 2,500-mile flight. But no one knew what became of the butterflies once they reached Mexico. Within the next few years, twelve more monarch roosts were discovered. They were all along the same mountain range where the Bruggers had made their find. Now the mystery was solved.
     The monarch’s stay in Mexico is just one part of an amazing life cycle. Every spring, in Mexico, female monarchs lay enormous numbers of eggs. One female may lay more than four hundred a month. She attaches her eggs to milkweed plants. The milkweed provides a perfect first home for the young monarchs. Because milkweed is poisonous to most creatures, birds and other butterfly enemies avoid it. But monarchs love milkweed. The eggs hatch in three to twelve days, and out come worm-like larve（幼虫）which feed on the milkplant. The poison does not hurt them. But it does have an important effect. It makes the monarch as poisonous the plant was. A bird that eats a monarch will become very sick—and never eat another one.
     After living for two weeks as larvae, the monarchs attach themselves to leaves. Then they spin cocoons(茧). After a week, the cocoons open and the butterflies emerge, soon to begin their 2,500-mile flight northwards. Many of them die as they pass through such southern states as Texas and Louisiana. But first they lay more eggs. After a few weeks, a new generation of monarchs is ready to continue the journey. They—or their children or grandchildren—will reach Canada, where they spend the summer.