Tennis is the only sport ___ I can play with you. 我寫which 答案that 為什麼不能用which? 謝謝><?

只要知道一個竅門，這種題目就是個送分題！

關鍵在 the only。

英語關係代名詞，只要先行詞部分有表示「全部、唯一、一些、某些、每個、許多」等，如 all、 only、any、anything、every 、everything、 no、nothing、some、something、few、little、many、much、the thing 這些字，以及最高級，如： the best、the most、the ＋～est 等，習慣上要用 that，不要用 which。

例如：
This is the best book that I have ever read. （最高級 the best book）
這是我讀過最棒的書。

That is all that I have to say. （表示「全部」 all）
我所要說的全部就是這些了。

There is something that I want to tell you. （表示「某件事」 something）
有件事我要告訴你。

「先行詞」的前面有最高級（如 the first, the only, the best）時，通常用that：
That's the most beautiful music "that" I have ever heard.
這裡的that可以省略。
請參見p.376 綿貫陽著《新觀念英文法》

你的句子有the only sport，所以通常是用that，而非which。

that, relative pronoun plural unchanged; used in defining, restrictive clauses; hence not preceded by a comma; yet often preferred to "which" for things="that",and often replaced by "that"; and often used in place of "which".---thing=sport=eg:-Tennis is the only sport(that)----defining clause.
which:-interrogative pronoun; which persons:-eg:-Which is taller?Which of the sport do you wish to play with?

Tennis is the only spot==that ; spot==which Yip