The old town has narrow streets and small houses ______ are built close to each other.
 
【正确答案】 D
【答案解析】 分析句子结构可知,先行词narrow streets and small houses在定语从句中充当主语,所以用关系代词that引导。故本题选D。关系副词where表示地点,作地点状语。