---Is that the small town you often refer to?

---Right, just the one _______ you know I used to live for years.

A．that

B．which

C．where

D．what

设a、b是一棵二叉树上的两个结点，在中序序列中，a在b之前的条件是（）。

A．a在b的右子树上

B．a在b的左子树上

C．a是b的祖先

D．a是b的子孙