单选题

　　[阅读选择]Calorie restriction dramatically extends the lifespan of organisms as different as worms and rodents. Whether this works in humans is still unknown partly because few people are willing to submit to such a strict diet.

　　According to the passage， we do not know whether humans will benefit from taking in fewer calories partly because______.

　　A、humans， worms and rodents are different.

　　B、most people are not willing to be put on a strict diet.

　　C、the effect is not known.

　　D、genetic changes in tissues can not be performed on humans.