A、By comparing their different ages. B、By counting the dead ones in the forests. C、By examining the marks made on them. D、By col

Monarch butterflies are a common summer sight in the northern United States and Canada. These large orange and black insects brighten parks and gardens as they fly lightly among the flowers. What makes monarchs 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 journey to the south. Those heading for Mexico go first for the Louisiana Mississippi area. They fly across the Gulf of Mexico into Texas. Once in Mexico, they settle themselves in one of about fifteen places in a mountain forest filled with fir trees. Each place provides a winter home for millions of monarchs. There are so many butterflies that they 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. So one flying north might lay eggs in Louisiana and then die. The eggs of that 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, will make their way back to Mexico and then restart the journey.
    Scientists learn about monarchs’ migration by catching and making marks on the insects. By researching a monarch with such a mark and noticing where it came from, the next scientist can get to know things like the butterfly’s age and its routing.
Questions 23 to 25 are based on the passage you have just heard.
23. Which is one of the places where monarchs spend the winter?
24. How can the routing of monarchs’ migration be learned?
25. What does the passage mainly discuss?