——is believed that an after-dinner walk a day keeps the doctor awayAthisBthatCitDwhich

——is believed that an after-dinner walk a day keeps the doctor awayAthisBthatCitDwhich

C
it 作形式主语,真正主语是 that an after-dinner walk a day keeps the doctor away （主语从句）
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C
it is believed that 是固定句式
意思是大家认为
类似的句式还有
it is said that 据说
it is reported that 据报道
都是以被动形式加that 从句出现