few/fewer/fewest，many/more/most修饰可数名词；little/less/least，much/more/most修饰不可数名词。
amount+不可数名词：an amount of通常接不可数名词单数，是单数；the amounts of/an amounts of+不可数名词，是复数，如：large amounts of money were……；也有amounts+可数名词复数的情况，如：The amounts of carcinogens……。
number+可数名词：The number of+可数名词复数，是单数；a number of/numbers of+可数名词复数，是复数；the numbers of+可数名词复数，是复数

amout修饰不可数名词
ron:"twice as many", "one-quarter as many", etc. are not only acceptable as idiomatic expressions but also extremely common in the english language.

m
amout修饰不可数名词

some这里是副词，表示大约，大概，有点，如：
I fund the exam some interesting. 有点好玩。
Some thirty people attended the funeral. 大约有三十人参加了葬礼。

as many指代前面提到的数量，twice as many, one-quarter as many，也可以接名词。
本题的as many指代4000，less than one-quarter as many=less than one-quarter 4000

倍数的比较可以是as/as，more than或x times sth或x times a number

可数名词4000，
排除点，many fewer
as many 那么多

youcuole... amout不可数不可以用fewer C错

ron:"twice as many", "one-quarter as many", etc. are not only acceptable as idiomatic expressions but also extremely common in the english language.
since this idiom is present in the correct answer, which was written by gmac, IT IS CORRECT.

fewer修饰可数名词

fewer修饰可数名词4000.
as many 那么多,一样多

amount 修饰不可数名词，不能修饰4000

(B) Correcet；补全省略即there are fewer than one-quarter as many (as there were at the height of their popularity in the late 1950s)； fewer修饰可数名词正确；one-quarter在此做副词修饰as many，后面不用再加of，类似于twice as many/much as
(D) the number指代不清；the amount指代不清，并且只能修饰不可数名词

amount 修饰不可数名词，不能修饰4000.
可数名词
there are fewer than one-quarter as many（那么多）

as many的用法(摘自Longman):
as many=a number that is equal to another number ?表示同样的数字

They say the people of Los Angeles speak 12 languages and teach just as many in the schools.
in as many days/weeks/games etc.

A great trip! We visited five countries in as many days (=in five days).
*: "twice as many", "one-quarter as many", etc.都是非常常见的用法，记住这样的表达

B项： as many = of many. E项it 无指代对象

(c) amount修饰不可数名词，不能用来修饰4,000（drive-ins）这个可数的量。

amount修饰不可数名词，不能用来修饰4,000（drive-ins）这个可数的量。

注意：题目中的some是adj表示大约的意思
some 4000==》其实指代的是4000 drive-ins ，是一个可数名词

ron:"twice as many", "one-quarter as many", etc. are not only acceptable as idiomatic expressions but also extremely common in the english language.
since this idiom is present in the correct answer, which was written by gmac, IT IS CORRECT.
end of discussion.

manhattan:I found the explanation of "as many" in Longman Dictionary:
as many=a number that is equal to another number
examples:
- They say the people of Los Angeles speak 12 languages and teach just as many in the schools.
- A great trip! We visited five countries in as many days.

drive-in n. 免下车餐馆,免下车电影院 drive-ins 是复数了...
at the height of sth 在sth的鼎盛时期