问题补充：
Old people must .
A. speak to polite
B. be spoken politely
C. be spoken to politely
D. be spoken to polite
答案：
A. speak to polite
B. be spoken politely
(答案→)C. be spoken to politely
D. be spoken to polite
解析：此题考查含有情态动词的被动语态的基本构成。情态动词+be+过去分词
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