The nature is full of mysteries. When autumn arrives in the United States and Canada, you can see one of nature's, greatest scenes - the yearly migration (迁徙) of the monarch butterflies (帝王蝶).
No other butterfly in the world migrates like the orange and black monarch butterflies of North America (A)Theycannot live for long periods in cold weather. So each autumn around October, the butterflies travel up to 3,000 kilometers south to (B) ______. Some even fly to the mountains of Mexico. Plenty of monarch butterflies almost block out the sky. And you can hear their wings beating.
The butterflies usually spend two months reaching Mexico. After five months of stay, they head back north in spring. But not one butterfly finishes the whole round-tip. They (C) ______ eggs along the way and die. Three generations of butterflies will live and die during the spring journey alone. The fourth and final generations of the year are born in early autumn. But the butterflies hardly rest before starting the long journey south again to Mexico. The most amazing thing about these monarch butterflies is that they always return to the (D) ______ trees in Mexico that their great-great-grandparents used the winter before.
What mysterious monarch butterflies! They remind us of our nation. Millions of people, generation after generation use their blood and lives to build up our happy life. So as teenagers, we should (E) ______.
