By the time Bob arrives in Beijing, we have already arrived here for two days.

A．we have been staying here for two days

B．it has been for two days we have stayed here

C．it is already two days before we have arrived here

D．we will have stayed here for two days

生物和社会因素对健康的共同作用可举例为

A．近亲结婚

B．车祸

C．糖尿病

D．吸烟

E．空气污染