题目

How would we travel without maps? It would be a bit adventurous to start from Oxford University to London Bridge if there wasn’t a map of the London Underground at each station. In fact, a lot of the early map-makers were adventurers and explorers, especially in the 15th and 16th centuries. So what did people do before there were maps? Well, it was quite easy to use natural signs like mountainsand rivers if you were travelling on foot or riding a horse. People took small boats down rivers and followed coastlines. And it was much more logical (合理的) to use time, not distance, to measure 在正方体ABCD-A1B1C1D1中，异面直线AC与DC1所成的角是 ．