We went through a period ______ communications we

We went through a period ______ communications we

We went through a period ______ communications were very difficult in the rural areas.

先把句子还原： communications were very difficult (in) a period in the rural areas. 首先，从语感上判断，如果period前不加in感觉读起来怪怪的，如果没有语感就完全从句子结构分析；第二，从句子成分判断，communications是主语，were是系动词，difficult是表语，然后句子需要状语，而名词不能做状语，只有介词短语可以做状语，所以必须是 in a period，不能是a period。
确定句子缺少的成分后，选择用that还是where，关系代词that, which, whose, whom, as, who在句中做主语，宾语，表语或者定语；where, why, when在句中做状语，所以选择where，而定于从句中where=介词+which，所以呢，最后选择in which

We went through a period ______ communications weWe went through a period ______ communications were very difficult in the rural areas
你把先行词（a period）带到后面的句子中看，是不是少了什么成分，少了就要补

带有介宾的从句引导词为which