21. It's a simple act of ________to always hold the door for the person behind you.
(A) courtesy
(B) frequency
(C) harvest
(D) impulse
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統計: A(24), B(4), C(3), D(5), E(0) #3988848
統計: A(24), B(4), C(3), D(5), E(0) #3988848