The letter is form my sister,_____is working in Beijing.

问题描述:

The letter is form my sister,_____is working in Beijing.
为什么用who,不用that、which

sister is working in beijing,所以用who,指代sister
用which的话 表示the letter is from my sister这句话is working in beijing.显然错的
that在定语从句中不出现在逗号后,所以that 也是错的