广告定位理论形成于（）
A.二十世纪六十年代
B.二十世纪七十年代
C.二十世纪八十年代
D.二十世纪九十年代
参考答案：B
They considered ______a computer, which was considered ______a great help in their work．
A．to buy; to be
B．buying; being
C．to buy; being
D．buying; to be
对于MCS-51单片机来说，一个机器周期是由（）个时钟周期构成。
A.1
B.2
C.4
D.6
