为什么Longmen Grottoes作主语谓语用复数

表示成双成套的名词，如trousers，shoes，glasses，compasses，socks，gloves，clothes等做主语时，谓语动词用复数。You trousers are much too long. 你的裤子太长了。Clothes keep people warm. 衣服使人暖和。

若上述名词背a pair of/pairs of修饰，谓语动词则根据pair/pairs的数来确定。This pair of compasses was bought in Shanghai. 这副圆规是在上海买的。