there are fewer than one-quarter as many，这里并不是than和as出现在一个比较里，as many在这里是固定词组，表示同样数量的，如：twice as many, one-quarter as many，We visited five countries in as many days。

fewer可数，less不可数，amount代不可数，number代可数

there are fewer than one-quarter as many，这里并不是than和as出现在一个比较里，as many在这里是固定词组，表示同样数量的，如：twice as many, one-quarter as many，We visited five countries in as many days。

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.

In general, you won't need the second half of that construction if you have already mentioned the data elsewhere in the sentence.
the earlier part of this sentence already mentions the # of drive-ins in the u.s. in the 1950's, so it is not stated again. since it is not stated as part of an actual parallel construction, you don't need the second "as".
--
examples:
there were once 20 shirts on this shelf, but, now, barely half as many are left. --> i already mentioned the data (i.e., 20 shirts) earlier in the sentence, so it's not mentioned again.

there are barely half as many shirts on this shelf [i]as there were last week.[/i] --> i didn't mention this comparison point earlier, so i'm mentioning it now.

amount ==》n .通常去修饰不可数名词（但是amount这个词本身是可数名词的）==》 a large amount of work, money, furniture //
Food was provided in varying amounts.
amount也可以用作v.总计 达到 Our information doesn't amount to much 我们的资料有限

"amount" is a construction that can only be used to refer to UNCOUNTABLE quantities -- i.e., continuous quantities / mass nouns, which are not divided into distinctly countable units.
for instance, you can have an "amount" of water, furniture, etc., but you CANNOT have an "amount" of people, items, etc.

this sentence is very clearly speaking about a countable quantity -- the number (not "amount") of drive-in theaters. since that's countable, any choice with "amount" can be eliminated immediately.

A. there are和less than的不可数矛盾，并且drive-ins应该是可数的
B. right! fewer than one-quarter as many就像twice as many表示同样数量的
C. that amount是单数名词，the amounts是复数，并且amount后面跟不可数名词，不能跟drive-ins这种可数名词。
D. 同C，the amount后面跟不可数名词，不能跟drive-ins。the number和the amount指代不明，但主要还是the amount不能接可数名词的问题
E. 同C，并且it看起来应该指代the number of drive-ins，但是其在前文没有出现不能用it指代

fewer可数，less不可数，amount代不可数，number代可数

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

When drive-ins were at the height of their popularity in the late 1950s, some 4,000 existed in the United States, but there are fewer than one-quarter as many.
在 1950 年代后期，当免下车进入最流行的时候，美国大约有 4,000 辆，但只有不到四分之一。

as many ：同样数量的
如：twice as many, one-quarter as many，We visited five countries in as many days。

【引用】
1.关于可数/不可数的判断，fewer可数，less不可数，amount代不可数，number代可数，排除AC
2.as many在这里是固定词组，表示同样数量的，如：twice as many, one-quarter as many，We visited five countries in as many days。，as many 没问题，B没问题
3.D选项，应为less than one quarter of the amount，排除；E选项，it指代不清楚

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

1.措辞，不能用Less，因为比较的是数字，是可数的，ADE错
2.idiom: ono-quater as many表示同样的数量。
同理还有twice as many.
3.一致，there are和Less,that the amount一看就不一致。AC错
4.the amount是指代不可数的，C就不一致，D的the number和the amount不能对比，CD错
fewer可数，less不可数，amount代不可数，number代可数

amount指代不可数，不能形容4000，D中the number 和 the amount都指代不清
as many在这里是固定词组，表示同样数量的，如：twice as many

A less 修饰不可数，且用less的话，前面要用there is
B 正确，没选是因为缺少一个as，实际上是省略用法
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
D the number指代不清；the amount指代不清，只能修饰不可数名词；the number is less没有问题，less和smaller都可以修饰数字。
E it没有指代对象，that amount指代4,000 drive-ins错。

[*]AMOUNT - only used to describe objects that CAN'T be counted. For example: Bravery, charisma, and water
[*]NUMBER - used to describe objects that CAN be counted. For example: Cars in the parking lot, puppies in the pound, and zits on my pubescent face

[*]LESS - used to describe objects that can't be counted
[*]FEWER - used to describe objects that can be counted

fewer可数，less不可数，amount代不可数，number代可数

注意可数与不可数； as many意思为同样数量的

less不可数名词 salt, love, money
fewer可数 ingredients, dollars, people =>排除 a d e

GMAT里面AMOUNT的用法解释是：amount is for undifferentiated masses such as sand or water，
因为4000 drive-ins是4000个不同的个体所以that amount不可以修饰4000.
that amount是指some 4000, 可是amount是代表不可数名词, some 4000是可数名词

as many意思为同样数量的

amount修饰不可数，

注意可数与不可数； as many意思为同样数量的