A、Under the river bed. B、On the floor of the sea. C、In the volcanoes. D、At the foot of the mountains. C短文说，钻石是在狭窄的火山筒被发现的。所以C正确。

The diamond is considered the most famous and valuable jewel in the world.
     [19] Diamonds were made as a result of great volcanic heat and pressure. A volcano is a mountain with a hole in the top. When a volcano is very active it sometimes explodes and causes great damage.
    Diamonds were pushed towards the surface of the earth—millions of years ago—by a number of great volcanic explosions. It is in the narrow volcanic pipes that diamonds are found. They are also found among the sand and stones of certain riverbeds, and in a few places on the floor of the sea, for they were washed down the mountainsides by the rain.
    Diamonds are very rare. There are not many diamond pipes or diamond-producing rivers in the world. During the last century, adventurers from Europe went to Brazil, because they had heard that there were diamonds in the River Amazon. Most of these early diamond miners died of illness or were lost forever in the great forests. But some returned home rich.
    The earliest known diamonds were found in India many centuries ago. The most recent and exciting discoveries have been made in Eastern Russia. But most of the world’s diamonds now come from the Congo, from Tanzania—which has the largest diamond mine in the world—and from South Africa.
19. What does the speaker say about diamonds?
20. Where can diamonds be found?
21. What happened to most of the adventurers from Europe?
22. According to the speaker, in which country were the earliest known diamonds found?