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This is a combinations thing (ie the order in which the members are picked does not matter). The number of ways in which 2 members can be picked is 190, so:
190 = ------------
which can be worked to:
380 = x*(x-1)
Then plug away! Since the answers are given in increasing value start with the middle one (C). You get 40*39 which is >1000 so then move "up" to a smaller answer. Anwers B ia also too big. Then finally to A. 20*19 = 380 so the answer is A. By starting in the middle you know which way to move ("up" or "down") and that way you only need to plug in 3 out of the 5 answers, saving a little time.