题目内容：

    Plant cloning has been an agricultural technique used by farmers and gardeners for centuries．Grafting(嫁接)is a common form of plant cloning．Many plants in nature actually clone themselves and reproduce asexually．

Cloned plants are much more predictable than normal plants, so their yield(产量)is more reliable．Cloned plants also reproduce faster, limiting the amount of time between planting and harvesting．It is often cheaper to produce seeds through cloning than through traditional methods．Plants can essentially be optimized so that farmers or individual growers always have the best seeds available．With more yield at a faster rate, farms can produce more food for more people while decreasing overall costs．

Cloning can be used to wipe out diseases that previously killed off the entire fields of crops．This would make plants immune(免疫的)to the kinds of diseases that farmers and gardeners around the world fear each year．Crop failures due to disease and virus could become a thing of the past．Also, plants that are near extinction can be brought back to life through cloning．

Through cloning scientists can develop“super”fruits and vegetables of superior nutritional quality．This could make for a healthier population overall．This is already being done through selective pollination(授粉), and genetic cloning could take it to the next level．

By plant cloning we have a population of identical plants and all these plants equally suffer from the same diseases, but in nature the weak would die and the strong would survive．While there are some dangers to consider with plant cloning, there are plenty of benefits that could prove vital to the propagation(繁衍)of the human species．

1.The underlined word“optimized”in Paragraph 2 can be replaced by“      ”．

A.enlarged B.produced

C.perfected D.examined

2.According to the text, cloned plants      ．

A.have lower yield than normal plants

B.have a short growth cycle

C.resist various kinds of diseases

D.need little care from people

3.what can we infer from the text？

A.The fruits produced by cloned plants taste much better．

B.There are more cloned plants than normal plants．

C.Plant cloning can disturb the laws of nature．

D.People should avoid cating cloned fruits．

4.What is the main purpose of the text？

A.To warn the negative effects of plant cloning．

B.To present different kinds of plant cloning．

C.To predict the future of plant cloning．

D.To inform us about the advantages of plant cloning．