51系列单片机40脚DIP封装的第10引脚功能是（）
A、串行输入口
B、串行输出口
C、外中断0输入
D、外中断1输入
参考答案：A
细胞分裂过程中，最先分裂的结构是（ ）
A．细胞核
B．细胞质
C．细胞膜
D．细胞壁
—The Summer Palace is wonderful. Have you ever visited any other interesting places?
—Yes. Also, we _______ to the Great Wall. [ ]
A. have gone
B. have been
C. had gone
D. had been
