It is difficult to study the depths of the Arctic Ocean, primarily due

Official Explanation:

Answer: E

For now, let’s keep out A because it uses the avoidable being. (Remember the usage of being should be avoided but it may not always be incorrect. If we don’t like any of the other options, we will go back and reconsider option A.) Option D does not have a verb, so let’s eliminate D.

This brings us down to B, C, and E. Comparing these three options vertically, we immediately spot the split between due to and because. Remember due to can only be used to replace caused by; it can never be used to replace because/because of. Try replacing due to with caused by in option B and you’ll realise that this makes no sense. So B goes out.

C and E both are grammatically correct. Now you are being tested on the meaning of the sentence. The original sentence is using primarily to modify because so as to suggest that the primary reason for the difficulty is the fact that the icy surface is difficult to penetrate. E conveys this same meaning whereas C (by using primarily to modify icy) distorts the meaning by suggesting that the surface is primarily icy. Thus, E is the correct answer.