You should make a rule to leave things you can find them easily.
A. it; where
B. it; then
C. that; there
D. this; when
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(答案→)A. it; where
B. it; then
C. that; there
D. this; when
解析：考查代词及状语从句。句中it作形式宾语，真正宾语为to leave things where you can find them easily；where引导地点状语从句，选A。