When drive-ins were at the height of their popularity in the late 1950s, some 4,000 existed in the United States, but today there are less than one-quarter that many.
there are less than one-quarter that many
there are fewer than one-quarter as many
there are fewer than one-quarter of that amount
the number is less than one-quarter the amount
it is less than one-quarter of that amount
由句子的前面部分,可以知道drive-ins是可数名词,所以后面不能用less,要用fewer,还有amount是用在不可数名词上的,也不能出现,所以答案是B。
另外: as many在这里是固定词组,表示同样数量的,如:twice as many
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