Medical research at the cellular level,_____research on the immune system,has been made possible through twentieth-century advanceds in techniques of genetic research.
A) which B)whereas C) such as D) is also
为什么选C,不选A呢?
11.In jazz music,a riff is a simple melodic figure,____and repeated several times.
A) usually one or two measures lasted
B) lasted one measure or two usually
C) is lasting one or two measures usually
D) usually lasting one or two measures
不是和and是并列关系吗?怎么不选B