His father often helps him _____ his lessons. [ ]
A. in
B. at
C. with
D. on
答案:C
OSI七层模型中的哪一层用于计算机间的标准化通信,保证实现数据包无差错、按顺序、无丢失、无冗余的传输()
A.网络层
B.表达层
C.传输层
D.应用层
一次造成职工和居民一百人及其以上的急性中毒事故属于( )。
A.特大伤亡事故
B.急性中毒事故
C.重大伤亡事故
D.特别重大伤亡事故