akash singhal wrote: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.

A. transplant, use them to test drugs and potentially toxic chemicals, and may study them
B. transplant, using them to test drugs and potentially toxic chemicals, and studied
C. transplant, used to test drugs and potentially toxic chemicals, and studied
D. a transplant, use them for testing drugs and potentially toxic chemicals, and for studying
E. a transplant, used to test drugs and potentially toxic chemicals, and may study them

OE C

Please Explain why 'used' is selected over 'use'.
[/b]

This question tests the concept of parallelism.

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

I hope now you get why used over use?

Put in the correct option:

Doctors hope that one day the body's master cells can be
1. directed to grow in organs or tissues appropriate for transplant,
2. used to test drugs and potentially toxic chemicals, and
3. studied to gain insight into basic human biology.

akash singhal wrote:Please Explain why 'used' is selected over 'use'.

A: DOCTORS hope that one day the body's master cells can be directed, USE them to test drugs
Here, the implication is that DOCTORS already USE the body's masters cells to test drugs.
Not the intended meaning.
The intention is to convey what doctors HOPE, as in the OA:
Doctors hope that one day the body's master cells can be DIRECTED...USED...and STUDIED.