[求助]易错语法题目精编中几道题目的解释

21.I will employ the man ______ they say is a fluent speaker of English.
A, who.
B, that.
C, which.
d. whom.

答案b，不能理解＝＝，另外顺道请教一下指代人时，who/that/whom的用法区别（请不要只说whom是宾格T T，不知道在什么情况下才该用whom，接在代词后的我知道）

26.By the time Smith (graduates) from (dental school), he will be (twenty-six) (years of age)

答案是b，可是错在那里？？@@

28.Kevin is (particularly) (fond of) cooking, and he (really) (cooks) delicious meals.

答案c，同样不知道为什么错＝＝，虽然其他部分也没有错误，但是也不觉得really用错了……因为我自己也肯定会这么说……

42.“john had the piano tuned today.”
“ was it_______?”.
A, out of tune badly before.
B, before badly out of tune.
C, badly out of tune before.
D, out of tune before badly.

答案是C，我选的是A，遵循副词一般后置的原则，不明白A错在哪里＝＝

49.“ I did not go to the party.”
“did _____ go to the party?”
a, many john friends.
B, many john's friends.
C, my sister boy friend.
D, a boy friend of my sister's.

答案D，这题完全不明白＝＝，莫非是不能用sb's xx，而必须用xx of sb's？？虽然读起来好像是用of比较顺啦……为什么？希望得到合理的解释。