This is _____ room, and it is beautiful and lovely.    [ ]

A. Betty and Sara

B. Betty's and Sara's

C. Betty and Sara's

D. Betty's and Sara

二叉树是结点的有限集合，这个有限集合或者为______，或者由一个根结点及两棵不相交的、分别称作为根的左子树和右子树的二叉树组成。