— When did Tom come to Qingdao？

— It was in July，2006 and he          a trip in China with his parents at that

time.

A．would take

B．had taken

C．was taking

D．had been taking

航天探测发现：金星周围的大气层中含有三氧化二碳(C₂O₃)，则C₂O₃中碳元素的化合价为[ ]

A.+1

B.+2

C.+3

D.+4