many copies of the best and healthiest plants.

　　It has been much more difficult to clone animals.Frogs have been cloned，but they died as tadpoles，never living long enough to become adult frogs.Mice，sheep and cattle have been cloned from embryos(胚胎)．But Dolly is the first healthy clone of an adult mammal.How did they do it?

　　(1)An unfertilized egg was taken from a sheep.

　　(2)The egg nucleus(细胞核)was removed.

　　(3)A cell was taken from the udder(乳房)of a pregnant(怀孕的)sheep.It was"starved"，so that it stopped growing.

　　(4)The two cells were combined into one and then implanted in a third sheep，where it grew normally.

　　(5)The clone was born.It was genetically identical to the pregnant sheep in Step (3)．

　　This sounds much easier than it actually was.To end up with one clone，the scientists started with 277 udder cells.And they needed in a way to make the egg cell accept a new nucleus.They did that by starving the cell in Step (3)．

　　It may soon be possible to clone many types of animals.Will we also be able to clone human organs，or even whole human beings? We have only begun to think about the morality of cloning technology.Maybe the question to ask is not whether we can clone humans，but instead，should we clone humans?

　　【语篇解读】　本文介绍了克隆技术及多莉羊的克隆过程。

　　4．Dolly is a sheep because she is grown from a cell of an adult female sheep.

　　A．that has no father

　　B．that has no mother

　　C．that has no parents

　　D．that has no tail

　　C　[推理判断题。从文中可以看出，多莉是从一只成年羊的细胞中克隆出来的，所以它没有父母，而是那只羊的一个翻版。]

　　5．Tell which of the following statements is TRUE?

A．Only plants can be cloned.