PPP提供了（）验证协议，因而比HDLC更安全。
A.PAP
B.MD5
C.CHAP
D.SHA
参考答案：A, C
In front of the Great Pyramid, Mr Lee stood _____ its great height, and asked many questions.
A．to gaze up at
B．to gaze down at
C．gazing up at
D．gazing down at
蓄电池放电后必须在多少h内及时补充充电（）
A、12h
B、24h
C、48h
