I posted 2 questions on purpose in order to make a striking comparison

In large doses, analgesics that work in the brain as antagonists to certain chemicals have caused psychological disturbances in patients, which may limit their potential to relieve severe pain.

(A) which may limit their potential to relieve
(B) which may limit their potential for relieving
(C) which may limit such analgesics’ potential to relieve
(D) an effect that may limit their potential to relieve
(E) an effect that may limit the potential of such analgesics for relieving

OA: E

Following the destruction of the space shuttle Challenger, investigators concluded that many key people employed by the National Aeronautics and Space Administration and its contractors work an excessive amount of overtime that has the potential of causing errors in judgment.
(A) overtime that has the potential of causing
(B) overtime that has the potential to cause
(C) overtime that potentially can cause
(D) overtime, a practice that has the potential for causing
(E) overtime, a practice that can, potentially, cause

OA: E

These two are really confusing, and I couldn't find an answer in the book of ManhattanGMAT Sentence Correction.

HELP! Please!!!

Thanks for clarifying why you posted two so we didn't tell you to split them up! :)

Generally speaking, they're both okay if used correctly. Follow "potential to" with an infinitive verb. Follow "potential for" with a noun. Know that they usually prefer an infinitive verb, where possible, rather than a gerund (a verb made into a noun). But you don't actually need to know how to deal with these idioms to answer the first one - they don't make you decide based on that fuzziness.

In the first one, A, B, and D have pronoun issues ("their" has multiple possible antecedents) and A, B, and C have modifier issues ("which" introduces a noun modifier, which must be placed next to the noun it modifies... and it's not).

In the second one, A, B, and C have modifier issues. "That" introduces an essential noun modifier, but the information following the word "that" is not essential in order to understand the meaning of the words coming before the word "that" - rather, the stuff after the word "that" makes an additional, but separate, point. Also, "that' introduces a noun modifier, but the noun before is not what they're trying to refer to - they're trying to refer to the fact that they work an excessive amount of overtime. That's a whole clause, not just a noun.

To know whether something is essential, try stopping the sentence at the word "overtime." Do you still understand everything that was written to that point? Sure. You've lost the meaning of the extra words, but it hasn't messed up the meaning of the words up to the word "overtime." Then look at choices D and E - they introduce "that" after the words "a practice." Can you stop at "a practice" now and still know what they're talking about? Nope.

Now, between D and E, you do have to know that they consider "has the potential for causing" wordier than "can, potentially, cause. Fewer words, yes, but also just using the verb form (since we're talking about an action, that's preferred to writing the verb in noun form).