April 1st is the day -is called April Fool’s Day in the west

答案：AB。空后面的是定语从句,而先行词是the day,所以关系代词用which,但是用that也完全是正确的,which/that在定语从句中充当句子的主语,所以AB都是正确答案。资料：下列场合不能用thata。在非限定性定语从句中,即逗号后的定语从句不用that引导,指人时用who或whom,指物时用which。b。在“介词+ 关系代词”结构中不用that,指人时用whom,指物时用which。This is the book about which we are talking。c。先行词为that,those时,关系词用which指物,who指人What’s that which you have got in your hand?d。两个定语从句,一个用了that,另一个则用which。Let me show you the novel that I borrowed from the library which was newly open to us。e。关系代词后有插入语时,只用whichHere are some stamps which – I think – you can take away。