Because ethylene dibromide, a chemical used to fumigate grain, was blamed for the high rate of nerve damage suffered by people who work in grain-processing plants, many such plants switched to other chemical fumigants 2 years ago. Since then, however, the percentage of workers at these plants who were newly diagnosed with nerve damage has not dropped significantly. Therefore, either ethylene dibromide was wrongly blamed or else the new chemicals also cause nerve damage.
Which of the following is an assumption on which argument depends?

A. If the new chemicals cause nerve damage, the nerve damage caused would be different from any nerve damage that ethylene dibromide may cause.
B. There are no chemical fumigants that are completely safe for workers in grain-processing plants.
C. If ethylene dibromide causes nerve damage, it does not take 2 years or longer for that damage to become detectable.
D. Workers at grain-processing plants typically continue to work there even after being diagnosed with nerve damage.
E. Workers at grain-processing plants that still use ethylene dibromide continue to have a high rate of nere damage.

I chose C, but basically I kind of guessed.

When in doubt use the negation technique on assumption question. What you basically do is negate the assumption and see if it destroys the argument, if it does then you have the right answer and if it does not or if it helps the argument then it is a wrong answer.

Answer Choice C says:
If ethylene dibromide causes nerve damage, it does not take 2 years or longer for that damage to become detectable.

Now negate the above assumption:
If ethylene dibromide causes nerve damage, it does take 2 years or longer for that damage to become detectable.

Now clearly if it does take 2 years or longer to detect damage then clearly ethylene dibromide was correct to be blamed and the new fumigant might have nothing to do with the new cases of nerve damage. When negated this statement destroys the argument hence its the correct answer.