选择与画线部分意思相同或相近的选项。

— Thanks very much for your help. 

— It's a pleasure. [ ]

A. That's right.

B. It doesn't matter.

C. Right.

D. You're welcome.

若二叉树中度为2的结点有15个，度为1的结点有10个，则有______个叶结点。

A．25

B．30

C．31

D．16