Explanation

Setting each expression equal to k gives a = k/2, b = k/3, c = k/6, d = k/9, e = k/12, f = k/18. Then a + b = 5k/6 and c + d + e + f = k(1/6 + 1/9 + 1/12 + 1/18) = 5k/12. Their ratio is (5k/6)/(5k/12) = 2 (B).