【the car _____ i had been driving to work was destroyed in the fire

坐在车上就是 in the car 这里是定语从句。你填的which指代 the car. 而且我是坐在车里开车的。就只能用in the car了。也就是in which.

因为乘车就是固定用法 in the car 啊，如果用 by 的话，应该是by car 。明白了吧。这个句子的主语是The car 带了定冠词the了，所以就用in which了。