说反了。先行词被序数词修饰都用that ,比如 It's the third time that you've arrived late this month.
只用who 不用that的情况:
1.关系代词在介词后面,比如 I sent thank-you notes to the relatives from which I received gifts.
2. 非限制性定语从句, 比如 He lives in a big house, which has a garden.
只用that不用who的情况:
1.先行词是 anything, something, nothing,everything等不定代词,比如 He never reads anything that is worth reading.
2.先行词是 all,much,little,none 或 被这些词修饰时,比如 You can take any seat that is free.
3.先行词被最高级,序数词修饰,或被 the first, very, the only, the same 修饰时,比如My book is not the only thing that is missing.