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. 