They are teachers and don’t realize _______ to start and run a company.

A．what takes it

B．what they take

C．what takes them

D．what it takes

因为燃气中常含有氨和硫化物，所以在燃气管网中最好不使用（）制作的阀门和阀门密封圈。

A、铝

B、铝或铝合金

C、钢

D、铜或铜合金