Five cloned pigs，whose organs are much less likely to be rejected(排斥) by a patient，have been born in the U．S.

More than 62,000 people in the U．S. alone are waiting to ____ donated hearts，lungs and so on.The number of human donors falls far short of ____.Pig organs are of a(n) ____ size to human organs，and some scientists hope they might be used to help meet the ____.But previous attempts to transplant pig tissue into humans have ____.

The five pigs ____ a gene that adds a sugar to the surface of pig cells.The sugar would ____ immune(免疫的) rejection of the tissue.“This advance provides a near?time ____ for overcoming the problem that there is not enough human organs for transplants，”says an expert.“This is the ____ gene for overcoming the ____ stage of rejection.”

____，scientists warn that much more work is necessary ____ organs from copies of the pigs could be transplanted into humans.Human genes will need to be added，to ____ rejection of the organ in the long term.There are also ____ that pig viruses could infect patients.

Cloning techniques were ____ to the production of the pigs.Genes can only be knocked out(去除) in a single cell.Cloning of these single cells then allowed the ____ of a whole animal in which the gene was knocked out in every cell.But the PPL researchers have ____ in knocking out only one copy of the gene.The team will now attempt to knock out both copies of the gene.

The team will also ____ tests to investigate whether a virus from the pigs could infect human cells.“Although a lot of the work is very ____，we’re still very far off being able to grow an organ，” says Julia，who is working on this project and quite ____ to creating similar knock?out pigs with researchers at the University of Missouri.

1.A.exchange? B．check

C．possess? D．receive

2.A.discussion? B．demand

C．doubt? D．distance

3.A.beneficial? B．identical

C．similar? D．certain

4.A.shortage? B．condition

C．satisfaction? D．argument

5.A.continued? B．failed

C．finished? D．paused

6.A.lack? B．include

C．make? D．change

7.A.leave? B．cause

C．block? D．destroy

8.A.supply? B．ambition

C．contribution? D．solution

9.A.only B．rare

C．key? D．safe

10.A.last? B．whole

C．next? D．early

11.A.However? B．Therefore

C．Besides? D．Finally

12.A.because? B．before

C．if? D．after

13.A.prevent B．ignore

C．judge? D．weaken

14.A.regrets? B．emotions

C．concerns? D．interests

15.A.simple? B．vital

C．unusual? D．basic

16.A.collection? B．application

C．reception? D．creation

17.A.delighted? B．succeeded

C．joined? D．believed

18.A.conduct? B．plan

C．design? D．study

19.A.perfect? B．strange

C．exciting? D．disappointing

20.A.opposed? B．used

C．devoted? D．suited