【一道英语完型】高手进!the drivers of _were constanly

题目

【一道英语完型】高手进!the drivers of _were constanly
There was not enough room for all the people on the sidewalk at the roadside,and they overflowed into the road,causing danger to their lives from passing cars and buses,the drivers of (which )were constantly blowing their horns (喇叭) to persuade people to get out of their way.
为什么要用which 不能用them them指代的是cars and buses,为什么不可以

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