Much hard ________ he tried, he failed to catch up with the first runner.
A. although
B. as
C. while
D. when
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I would never speak to someone like that ________ they said something unpleasant to me.
A. even if
B. so that
C. as if
D. ever since
单片机AT89C51要使用自带的ROM存储器，则EA引脚（）
A. 必须接地
B. 必须加+5v电源
C. 可悬空
D. 以上三种视需要而定
外部扩展存储器时，具有第二功能的端口是（）
A. P0
B. P1
C. P2
D. P3
单片机上电复位后，PC的内容是（）
A. 0000H
B. 0003H
C. 000BH
D. 0800H
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