有以下物质（1）干冰，（2）石灰石，（3）铜片，（4）液氧

属于混合物的是______（填主要成分化学式，以下均填化学式），属于氧化物是______

属于由分子构成的单质是______．

My friend asked me ______ when he met me at the school gate. [ ]

A. who was I waiting for

B. who I was waiting for

C. who am I waiting

D. who I am waiting