单项选择题

A.R-CH=CH-CH2-X〉R-CH=CH-X〉RCH2-X
B.R-CH=CH-X〉R-CH=CH-CH2-X〉RCH2-X
C.R-CH=CH-X〉RCH2-X〉R-CH=CH-CH2-X
D.R-CH=CH-CH2-X〉RCH2-X〉RC6H5-X