—I haven’t seen Tom for years. Any news about him?

—He          in a toy factory for two years. Now he’s working on a farm.

A．works

B．worked

C．has worked

D．had worked

采掘工作面的进风流中，氧气浓度不低于20%，二氧化碳不超过（）。

A、20%

B、15%

C、0.5%

D、1%