–Has your father returned from Africa yet?

–Yes, but he _____here for only three days before his company sent him to Australia.

A．has been

B．will be

C．would be

D．was

空洞洞壁的成分可以是（）

A.坏死组织

B.肉芽组织

C.纤维组织

D.肿瘤组织

E.正常组织