求助：TT GWD 4-11

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 two 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 the argument depends?

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

答案是C B为何错？C取非的意思是ED需二年或以上才查出，对原文结论行不成削弱，因为还是有可能要么ED要么new chenmical cause nerve damage。为何是C?

read the last sentence again: Therefore, either ethylene dibromide was wrongly blamed or else the new chemicals also cause nerve damage.