单选题 A recent survey of problems on health______that outdoor exercise______of great importance to people's health.
A. have shown ; are B. are shown ; are
C. has shown ; is D. is shown ; is

【正确答案】 C
【答案解析】句意：关于健康问题的一项最新调查显示，户外锻炼对保持健康很重要。句子的主语是survey且是主动语态，所以选项C正确。