—  You should have thanked her before you left.

—  I meant ____, but when I was leaving I couldn't find her anywhere. [ ]

A. to do    

B. doing  

C. to    

D. doing it

串的长度是( )。

A) 串中不同字符的个数
B) 串中不同字母的个数
C) 串中所含字符的个数且字符个数大于零
D) 串中所含字符的个数