—How long 　　　　 in Shanghai？

—Just the weekend. I've to be back for an important meeting to be held next Monday.

A．have you stayed

B．are you staying

C．did you stay

D．do you stay

BJ008自吸过滤式呼吸防护器，用于在空气中有害浓度不高，且空气中的含氧量不低于（）的场所。

A.13%

B.15%

C.20%

D.18%