— I can't find my _____________.
— You're wearing them.[ ]
A. books
B. boots
C. dress
答案:B
用反证法证明“a<b”时应假设( )
A.a>b
B.a≤b
C.a=b
D.a≥b
绘图题:画出PCM数字电话时分复用结构示意图。