电缆隧道应有充足的（），并有防水、防火、通风措施。
A.空气
B.照明
C.氧气
D.防护
参考答案：B
--Did you listen to the report last Friday?
--- Yes, it was about the shortage of migrant workers. That was the first time I______ about the subject.
A．heard
B．was hearing
C．would hear
D．had heard
下列各组运算，结果正确的是( )．
A．3a +3b =6ab
B．—2x —2x =0
C．9x—6x =3
D．3y 2—y 2="2y" 2
