A、In the volcanoes. B、On the floor of the sea. C、Under the river bed. D、At the foot of the mountains. A推理判断题。第四段首句提到：Diamonds we

The diamond is considered the most famous and valuable jewel in the world.
    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 by a number of great volcanic explosions millions of years ago. It is in the narrow volcanic pipes that diamonds are found. They are also found among the sand and stones of certain river beds, and in a few places on the floor of the sea for they were washed down the mountain sides by the rain.
    Diamonds are quite 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. Many of these early diamond miners died of illness or went 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 word--and from South America.