分组网和分组网互联必须按照()建议规定。
A.X.75
B.X.21
C.X.28
参考答案:A
His mother looked very tired,for she _____ the house herself all day.
A.had cleaned
B.has been cleaning
C.had been cleaning
D.has cleaned
普通螺纹包括粗牙螺纹和()螺纹。
A.英制
B.梯形
C.细牙
D.锯齿形