In September 2008,China’s key milestone in space travel____when Zhai Zhigang conducted

the first space walk.[ ]

A.reached

B.was reached

C.has reached

D.had been reached

原油的化合物主要是由碳元素和（）组成，是以烃类化合物形式存在。

A.氢元素

B.烷烃

C.烯烃

D.硫