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I'm going with (E). We need to start with one factor before we say it's the main factor aside from others. Also, the inflation is a problem that has exsited in the past and stretching to the present, so we need a present perfect 'has been'.
(C) made the mistake of implying that there is currenty no inflation (happened in the past but not in the present)
If you don't have "other", then you are comparing the increases to everything else and itself
Ex: China has a population that is larger than that of any other country
In the above, "other" means that China's population is compared to that of all countries excluding its own. This is a concept often tested on the GMAT.