1. He reached Beijing in 1998, ______, some time later, he became a professor.
没错,答案填 where,定语从句所修饰的先行词就是 Beijing。在非限制定语从句中,判断先行词的最直接方法就是看句意。在此句中,有可能成为先为先行词的有两个:一个是 Beijing,一个是 in 1998。但根据句中的 some time later 可知,先行词不太可能是 in 1998(否则意思不通),所以确定先行词是 Beijing。
2. 像下面这样改写是不是更好呢?就这个单句来看,你的改写有可能“更好”,但在一定的上下文中,就得考虑前后连贯和衔接。
In 1998, he reached Beijing, where he became a professor some time later.