21,24,30,33,39,

题目

21,24,30,33,39,
下个数是?
答案中没有42
只有A. 51 B. 49 C. 47 D. 45

答案

A
24=21+2+1
30=24+2+4
33=30+3+0
39=33+3+3
=39+3+9
answer is 51