It is bad manners to an invitation without a reason.
A. break down
B. settle down
C. turn down
D. calm down
网友回答
A. break down
B. settle down
(答案→)C. turn down
D. calm down
解析：考查短语动词词义辨识。题意：无故拒绝邀请是不礼貌的。break down：（机械）抛锚；（人的精神）垮掉；settle down：定居；turn down：拒绝；calm down：冷静下来。故C项正确。