GWD17-33

33. GWD17-Q33:

Doctors hope that one day the body’s master cells, called stem cells, can be directed to grow in organs or tissues appropriate for transplant, use them to test drugs and potentially toxic chemicals, and may study them to gain insight into basic human biology.

1. transplant, use them to test drugs and potentially toxic chemicals, and may study them
           
2. transplant, using them to test drugs and potentially toxic chemicals, and studied
           
3. transplant, used to test drugs and potentially toxic chemicals, and studied
           
4. a transplant, use them for testing drugs and potentially toxic chemicals, and for studying
           
5. a transplant, used to test drugs and potentially toxic chemicals, and may study them

答案是C

为什么E不对呢

E后半句MAY STUDY THEM不可以看做是DOCTORS后边的HOPE的并列吗??

C后半句的STUDIED和前半句里的HOPE也不同时态的呀

哦，一语道破,我脑子不转了

谢谢了