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

【专家解说】：B
考查主谓一致。A survey of the opinions of experts中中心词是survey调查，抽象名词，不可数，故谓语动词用单数。下文，主语是three hours of outdoor exercise，中心词是outdoor exercise，抽象名词，不可数，故谓语动词用单数。选B。