急---為什麼要用that？

為什麼要填that 而不是which 或for ?

Because "that" used in the sentence (空格處) should be a conjunction introducing an appositive clause, but not a relative pronoun (if it is the case, "that" can be replaced by another pronoun "which") introducing a relative clause.

"for" is a preposition, and must be followed by a noun or noun phase or gerund phrase.

先要說明用that是由於後句的全句(共兩句都是)是一句完整句字
online shopping has experienced a recent boom worldwide,
and online businesses are making huge profits.
作為前文解釋的"理由/結論"的"one reason"(一個理由)
寫法是表示這兩句便是reason的結果/效果。

前句的That is one reason是引詞，所以要連接這引詞和後面完整的句子，唯有用that作為前後整句的連接詞以連接獨立成句的句子。

理論上用which亦是連接詞，只是which以後要說明which前的內容(字詞)的解釋而不是等同那意思。這句的前句內容是one reason，假若以which作連接詞後句的內容便是解釋或詳述作者認為這是one reason的理由而不是這reason的結果。作者只是寫後文(兩句其實並不是那one reason而是證明那one reason的後果)。因此不能寫which接連。

For是可解作because以表示"原因"作為連接詞。如上所解，後句的內容(兩句的獨立的內容是果而不是因)，所以後句並不是解釋That is the reason because 作者指出這是reason的原由，反而後句的內容只是以on-line shopping及on-line business引證是前文想說明的一種證據。

That後面接子句，而for後面接名詞片語戎單字， online shopping has experienced a recent boom worldwide, and online businesses are making huge profits.是以and連接二個子句，所以用that。若寫成 that's one reason for online shopping and online businesses. (Ok)當that名詞子句當受詞可省略，如 You don't know(that) the oil price has dosed since Jan.
再更深入研究for放置於句首可當連接詞，功能係副詞性質轉折語，例: For this was what the notice promised. =therefore,this was what the notice promised.

"that"字做冠詞online shopping has experienced a recent booming worldwide, and online businesses are making huge profits. Yip