________ made the school proud was ________ more than 90% of the students had been admitted to key universities.

A．What；because

B．What；that

C．That；what

D．That；because

现有关键码值分别为5、10、15、20的4个结点，按所有可能的插入顺序去构造二叉树。这些二叉树排序中有几棵是最佳二叉排序树

A．6

B．5

C．4

D．3