指示代詞that，下題為何不用which－jennin64

樓上大大恐怕沒說到重點.

Yet today, the achievement gap between the poor and the non-poor is
twice as large as ( ) between black and white students.
(1)it (2)one (3)that (4)which

英文的語言習慣很注重避免重複.

已出現的名詞(事物), 以相同結構再次提到時, 單數用that, 複數用those.
本句中的the achievement gap (單數複合名詞)再次提到時, 以that代替,
相同結構:
xxx between A and B
that between C and D
故正解是 (3)that.

天成翻譯社

然而現今，窮人與非窮人間的成就差距，兩倍於黑人學生與白人學生間(的成就差距)。