itwasmorethanfifteenyearsago---ienteredthelaboratoryofProifessora.whenB,thatCinwhichD.since用when为何不可

Question

it was more than fifteen years ago ---i entered the laboratory of Proifessor
a.when B,that Cin which D.since
用when为何不可

Answer 1

这里应该是强调句,所以用that.more than fifteen years ago是被强调部分.去掉it is··· that仍是一个完整的句子.如果用when的话,即理解为前面more than fifteen years ago的定语,而后面又没有其他成分的话,这个句子就是不完整的."I entered the laboratory of Professor more than fifteen years ago"才是一个完整的意群.