due：(车船)预定应到的;预期 sure：确定的
The early train is due to leave at 5 in the morning.You'd better get to the station by 5:30 at latest.早班火车预计早上5点发车,你最迟5:30分要到火车站.
by：到 by 5:30 到5:30分 是指5:30分这个点
around：大约,左右 around 5:30 可以是5;30之前,也可以是5:30之后,句子的意思是火车预计5点发车,你最迟5:30必须要到车站才可以赶上火车.所以选 due 和 by.
答案供参考,