After he left college , he was employed in an international company and _____        there ever since .

A．worked

B．has worked

C．had worked

D．was working

丙烯产品中含有大量丙烷，丙烯塔的调整操作中正确的是（）。

A、增加回流量

B、增加丙烯出装置流量

C、适当提高塔压

D、减少丙烷出装置流量