Tom and Mary, ______ are playing in the garden, are very naughty.
(A) who　　
(B) that　　
(C) whom　　
(D) which　　

1. 先行詞為人
who（主格）＋V
whom（受格）＋S＋Vt / S＋Vi＋prep
whose（所有格）＋N＋(S)＋V

2. 先行詞為物或事
which（主格）＋V
which（受格）＋S＋Vt / S＋Vi＋prep
whose / of which（所有格）＋N＋(S)＋V

3. that 可以代替 who, whom 及 which。

4. 先行詞為獨一無二的人，物或事時用補述用法，要用逗號撇開，不可用 that；反之則用限定用法，不須逗號撇開，作受格時 whom 或 which 或 that 可省略之。

例句：

1. 上個禮拜三搶劫銀行的嫌疑犯仍然逍遙法外。
The suspects who/that robbed the bank last Wednesday are still at large.

2. 姑媽送給我一只她在瑞士買的錶。
My aunt sent me a watch (which/that) she bought in Switzerland.

3. 大衛是我的小學同學，昨天我在街上巧遇他。
David, whom I met by chance yesterday, was my elementary school classmate.

from http://www.nani.com.tw/slearn/sleng/eng_d/Chapter09/Chapter09_4.htm