You can make a by turning off the light after you leave the house.
A. difference
B. decision
C. promise
D. mistake
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(答案→)A. difference
B. decision
C. promise
D. mistake
解析：考察固定搭配。Make a difference起作用，与影响；make a decision做出决定；make a promise做出允诺；make a mistake犯错误。句意：你离开房间以后把等关掉也能起很大的作用。根据句意说明A正确。