HF系统的工作频率范围().
A.2.0—30MHz
B.2.0—23.999MHz
C.2.8—23.999MHz
D.23.999—30MHz
参考答案:C
9.―Your job ____ open for your return.
―Thanks.
A.will be kept
B.will keep
C.had kept
D.had been kept
MgO是碱性氧化物,在炉渣中能起脱硫作用。()