A is incorrect because 'pursue what they had not considered......' is awkward.
and also 'better control of the other infections' or 'eradication of the other infections' is awkward.

Skywalker wrote:Hi,

Sort of completely missed out on this one.....pls let me know why is the [spoiler]OA = C and not A[/spoiler]

The success of the program to eradicate smallpox has stimulated experts to pursue what they had not previously considered possible -- better control, if not eradication, of the other infections such as measles and yaws.

A. what they had not previously considered possible -- better control, if not eradication, of the other infections such as
B. what they had not previously considered a possibility -- better control, if not eradication, of such infections like
C. something they had not previously considered possible -- better control, if not eradication, of such infections as
D. something not considered a previous possibility -- better control and perhaps eradication, of other infections such as
E. the possibility of what they had not previously considered possible - better control and possibly eradication of infections like

The issues:

1. The following idioms are correct:

-> Such X as Y
--> Such as X and Y
--> Such as X, Y, and Z

==> This kills the options B and E because they have wrongly used "infections like X and Y"

2. "something not considered a previous possibility" is changing the intended meaning of the sentence.

Previous possibility---is not like "previously considered possibly"

==> This kills D.

3. "of the other X such as .." is not correct. In this case the is a problem.

Besides, "to pursue + NOUN" should be the correct way. But in the option A, pursue + Clause has been used.

Answer is C.