答案 D
解析 主旨大意题。根据文章最后一句“We can learn a lot from the elderly,and they can often benefit from teenagers,too.”可知选D。
B
(建议用时8′)
British scientists have done something that many people thought was impossible:they created the first clone of an adult mammal(哺乳动物).The clone,named Dolly,is a sevenmonthold sheep,grown from a cell of an adult female sheep.
What is a clone?A clone is a copy—a genetic copy.Genes are the instructions for life.They tell a cell what to do and how to do it.Genes make each individual plant or animal different and unique.But a clone has the same genes as the original organism(生物体).
Cloning is not new.Plants have been cloned for centuries.New plants are often grown from cutting from another plant.Also,plants like pineapples,strawberries and carnations (康乃馨) are sometimes cloned.One benefit of cloning is that we can make 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.
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