GWD-17-33:

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

My key is C. Anyone go for A (least evil).

For C, what part does "used to test" and "studied to gain" form parallelism? C looks awkward.