cars and buses can only move slowly when there is a lot of this on the road

cars and buses can only move slowly when there is a lot of this on the road
cars and buses can only move slowly when there is a lot of on the roads
cars and buses can only move slowly when there is a lot of this on the roads中的this指什么？选项为：swans，traffic，a book，a lamp，chopsticks。比如举个例子：you take it when you are not feeling well,which helps you feel better.答案是medicine（药）.再举个these insects have beautiful wings and fly from one flowers to another.答案butterflies。我觉得选项中没有对的，（不是翻译啊）
是traffic。谁先回复我给谁采纳为最佳答案