Insert the missing number 2, 6, 12, 20, 30, 42, 56, (....)
Answer: C
The pattern is 1 x 2, 2 x 3, 3 x 4, 4 x 5, 5 x 6, 6 x 7, 7 x 8.
So, the next number is 8 x 9 = 72.