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.
- transplant, use them to test drugs and potentially toxic chemicals, and may study them
- transplant, using them to test drugs and potentially toxic chemicals, and studied
- transplant, used to test drugs and potentially toxic chemicals, and studied
- a transplant, use them for testing drugs and potentially toxic chemicals, and for studying
- a transplant, used to test drugs and potentially toxic chemicals, and may study them
请问为什么选A，不选C呢？ A 的them没有指代不清的问题吗？可以指cells或者organs,tissues.而C就没有这个问题。 请哪位高人给讲讲，多谢了 | 
