is today Britain used to be three different countries,and over many years the three countr

is today Britain used to be three different countries,and over many years the three countries became one.
A,It B,What C,Where D,As

解析：通过分析句子结构，可知这是一个复合句，其中包含一个主语从句，且引导词在从句中作地点状语，故用Where 。

哦，回答的不错