51系列单片机40脚DIP封装的第17引脚功能是（）
A、计数器0输入
B、计数器1输入
C、片外RAM写控制
D、片外RAM读控制
参考答案：D
— Catherine, I have cleaned the room for you.
— Thanks. You _____ it. I could manage it myself. [ ]
A. needn't do
B. needn't have done
C. mustn't do
D. shouldn't have done
