It is generally said that radiations emitted by cell phones

As (A), (B), and (E) are clearly out of scope here, we need to look more closely at (C) and (D).

(C): Even if a majority of those who use hearing aids have never used cell phones, this proves that there other causes besides the usage of cell phones that can lead to hearing loss. However, this still does not weaken the assertion that cell phones can cause hearing damage. This is therefore not a good enough choice. Reject.
(D): Although this choice becomes the automatic answer through elimination, we should also verify if this option holds logically. The stimulus states that cell phone usage leads to immediate hearing damage, and further concludes that using a cell phone may lead to total loss of hearing. However, if (D) is true, and 90% of telephone operators have been using cell phones for years, this weakens the stimulus' conclusion. If cell phone usage did cause immediate hearing damage, and prolonged usage led to total hearing loss, these telephone operators couldn't possibly have been using cell phones for years (as if their hearing was damaged after some use, they could not continue to use cell phones). This option therefore weakens the conclusion and is the correct choice.

(D) it is.