A. if B. so
C. until D. or
［答案］ A.
［析］ 本句译为：如果你将心放在学习上，你就会将英语学好。这里的语法现象是从句中用一般现在时表示将来。
21 I won't let you in ＿＿＿ you show me your pass.
A. until B. for
C. since D. because
［答案］ A.
22 She didn't go to school ＿＿＿ she was ill.
A. why B. because
C. where D. but
［答案］ B.
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