Passage2Questions 21 to 25 are based on the following passage:In the past oysters (牡蛎) were raised in much the same way as farmers raised tomatoes—by transplanting them. First, farmers selected the oyster bed. Next, they “planted”fertilized oyster eggs, which within two or three weeks hatched into larvae (幼虫). The larvae drifted until they attached themselves to the clean shells on the bottom. There they remained and in time grew into baby oysters called seed or spat. The spat grew larger by drawing in seawater from which they derived particles of food. Before long, farmers gathered the baby oysters, transplanted them in other waters to speed up their growth, then transplanted them once more into another body of water to fatten them up.Until recently the supply of wild oysters and those crudely farmed were more than enough to sat isfy people’s needs. But today the delicious seafood is no longer available in abundance. The problem has become so serious that some oyster beds have vanished entirely.Fortunately, as far back as the early 1900’s marine biologists realized that if new measures were not taken, oysters would become extinct or at best a luxury food. So they went to work. But they did not have the proper equipment or the skill to handle the eggs. They did not knowwhen, what, and how to feed the larvae. And they knew little about the predators (食肉动物) that attack and eat baby oysters by the millions. They failed, but they kept at it and refused to give up in the face of difficulty. Finally, in the 1940’s a significant breakthrough was made.Question:The main idea of the passage is ______.

A. the cultivation of oysters
B. the discoveries made by marine biologists
C. the threatened extinction of marine life
D. the varieties of wild oysters