Histocompatibility antigens that attack foregn tissue in the

Histocompatibility antigens that attack foregn tissue in the body cannot have been .through evolution expressly to .organ transplantation; on the contrary,they have been found to facilitate many essential biological functions.
A designed ...retain
B produced ...aid
C developed ...enhance
D selected ...promote
E conserved ...foil
我认为,on the contrary 前后句意应该相反吧,前句有cannot,那就是说明第二空应该填和后一句facilitate同义的词才对啊,还是说应该这样理解,分号前后态度相同,那就是cannot have been to ...和 have been ...facilitate 一致,所以说第二空应该填facilitate的反义.该怎么理解啊,分号是什么含义