Question 19 is based on the passage you have just heard.

Passage Two

    Monarch butterflies, the large orange and black insects, are a common summer sight in the northern United States and Canada. They brighten parks and gardens as they fly among the flowers. (19)What makes monarch butterflies particularly interesting is that they migrate--all the way to California or Mexico and back. They are thought to be the only insect that does this. Every year in the late summer, monarchs begin their migration to the south. Those heading for Mexico go first for the Louisiana-Mississippi region. Then they fly (20) across the Gulf of Mexico into Texas. Once in Mexico, they establish themselves in one of about 15 sites in a mountain forest. Each site provides a winter home for millions of monarchs. The butterflies are so numerous that they often cover entire trees. When spring comes, they begin their long journey north. The question is often asked whether every butterfly makes the round trip journey every year. And the answer is no. The average monarch lives about nine months. (21)So one flying north might lay eggs in Louisiana and then die.

    The eggs of that following generation may be found in Kentucky; the eggs of the next generation may end up in Wisconsin or Michigan. The last generation of the season about the fourth may make the journey back to Mexico and restart the cycle. Scientists learn about monarch butterflies’ migration by capturing and placing identifying tags on the insects. By recapturing a tagged monarch and noting where it came from, the next scientist can figure out things like butterfly’s age and its routing.

19. What is the unique about monarch butterflies according to the speaker?

正确答案：C

解析：短文中提到帝王蝶每年夏末便开始它们去往南方的迁徙，它们被认为是唯一一种这样做的昆虫。