A survey of the opinions of experts ________ that three hours of outdoor exercise a week ______...

【專家解說】：B
考查主謂一致。A survey of the opinions of experts中中心詞是survey調(diào)查，抽象名詞，不可數(shù)，故謂語動詞用單數(shù)。下文，主語是three hours of outdoor exercise，中心詞是outdoor exercise，抽象名詞，不可數(shù)，故謂語動詞用單數(shù)。選B。