12. The principal gave a speech to the freshmen, only a few of______ were actually paying attention.
(A) who
(B) whom
(C) that
(D) them

WHO [主要用在問"誰", 而回答多數為人物, 如名字, 職稱等, 多用在subject] 
1. (用作疑問代名詞)誰,什麼人 
Q: Who has borrowed my pen? 
--> A: Mary. / The Doctor. / My teacher...etc. 
2. (用作限定關係代名詞)...的人 
The girl who spoke is my best friend. 
--> 這裡的who是指The girl. 
3. 用作非限定關係代名詞)他;她;他們;她們 
I went to the movies with Mary, who I think you once met. 
--> 這裡的who是指Mary. 

WHOM [多用在object上, 回答多用第三人稱, i.e. him, her, them等] 
1. (用作疑問代名詞) 誰,什麼人 
I know who you are and whom you represent. 
--> 這裡的whom的意思是"I represent him/her" 
2. (用作限定關係代名詞)...的人 
I'll give my vote to the man in whom I have faith. 
3. (用作非限定關係代名詞)他,她;他們,她們 
My new neighbor, whom I introduced to you the other day, had a heart attack last night. 

此題屬於第三項用法 

校長給新生講了話，真正在意的只有______幾個