— What does your father do?
— He is ______________.[ ]
A. clerk
B. a clerk
C. clerks
答案:B
资源利用的外部性
对于图中错误的是
A、开关掷于1是开路
B、开关掷天2是通路
C、开关掷于3是通路
D、开关掷于3是短路