The seventeenth century was one ___ many advances were made

The seventeenth century was one ___ many advances were made in science.
A. that B. which C. where D. when
答案是 when 或是in which.
按照我以前的思维我不会觉得有什么不对,只是最近又系统的看了一遍定语从句的理论知识,发现上边有一条是先行词是one的时候,定语从句要用that引导.这就有点搞不明白了.

（先行词是one的时候,定语从句要用that引导）这一规则不能单独来看,定语从句引导词的选择,首先要看句子成分.也就是我们通常所说的：考查定语从句中有没有缺失的句子成分,如果缺主语或宾语,那就要选择合适的关系代词,比如which或that.who.之类的.有些句子which和that是可以互换的,有些句子就不行,就象你在书上看到的,如果先行词是one或者有firsrt修饰时就只能用that.当然只能用that不能用which的情况还很多,我就不列举了.
再看上面这一题,定语从句中是不缺少主语和宾语的,那么引导词就只能选择关系副词,比如when,where.先行词one指代的是前面的“世纪”那么就只能选when了.因此不考虑关系代词that.