如果草原上放养的牲畜太多，就会严重破坏草场植被，造成土地沙化，草场就很难恢复原样了，这说明生态系统              能力是有限度的。

---- How long ____ at this job?

---- Since 1990.

A．were you employed

B．have you been employed

C．had you been employed

D．will you be employed