Tripura TET Paper 1 2018 Solved Paper

Directions : In Question each sentence has four underlined words or phrases. The four underlined parts of the sentence are marked as (A), (B), (C) and (D). Identify the one underlined word or phrase that must be changed in order for the sentence to be correct.
Air pollution, together with (A) /littering, are (B)/ causing many  problems(C)/ in our large,industrial cities today.(D)