In the following question, select the missing number from the given alternatives :

2, 6, 4, 9, 8, 13, 16, 18, 32, ?

There are two alternating series : (i) 2 × 2 = 4 4 × 2 = 8 8 × 2 = 16 16 × 2 = 32 (ii) 6 + 3 = 9 9 + 4 = 13 13 + 5 = 18