单选题 They also helped build the vast network of railroads that ____ important trade centers essential to industrial grented.
【正确答案】 D
【答案解析】首先可判断出此句是关系代词that 引导的定语从句。先行词是network ofrailroads，从句中缺少谓语动词，故可排除C。根据主句的时态，应用一般过去时，故选D。