sat语法2题 1.The team of flight mechanics,_four who_are certifi

sat语法2题
1.The team of flight mechanics,_four who_are certified electricians,work on all of the interior mechanical malfunctions reported by the flight crews.
A.four who
B.four that
C.four of whom
D.four which
E.four of which 此题选c,为啥一定要of whom?直接four who不行吗?
2.If (you) plan to participate in a marathon,you need to realize that many athletes,even (those) in top physical condition,find it difficult to finish (such) a long race; by the end of a marathon you (will run) 26 miles.(No Error)
此题选第4个,即will run错了.答案说要改成will have run,因为是by the end of .为啥要用将来完成时?