----- you are here,please stay with us for the night.空格之上该填什

_____ you are here,please stay with us for the night.空格之上该填什么?
选项有：since because when if
重点要区分的是since和if,究竟有什么可以很容易就分开的,我觉得两者都可以放进去,只是意思有所不同.但是参考答案是 since 请问为什么?