A. the numbers指代不清
B. Correct；came off作过去分词修饰trucks；that number指代前面总和的469000，如果是the number，没有指代对象，似乎说是the number of trucks。
C. the number指代不清；且alone比by themselves好，句意为强调"单单是卡车就有那么多了", 而不是强调"卡车它们自己怎么怎么样"，所以不用themselves。
D. twice结构不完整，少一个as，by themselves错误同C。
E. 错误同D。
补充说明
倍数表达方法总结：
"A + be + 倍数 + as + 计量形容词原级 + as + B
"A + be + 倍数 + 计量形容词比较级 + than + B"
"A + be + 倍数 + the + 计量名词 + of + B"

A.numbers doesn't make sense. We're talking about a single number, not about more than one number.
B.that number指代469,000，alone表示单独算trucks
C expression " by themselves" conveys the idea that the trucks came off the assembly line automatically by themselves

by themselves是指卡车出售时没有附带配件，即单车出售，与alone意思并不相同。

A. the numbers错误， twice后面要跟单数的number，即twice the number；但是改为the number后，指代特定的数字还是用that number更为明确
B. Correct； that number指代前面总和的469000，如果是the number，没有指代对象，似乎说是the number of trucks

题目释义： In 1914 a total of 469,000* cars and trucks were……， but in 1929 almost twice that number* of trucks alone …….
1914年cars和trucks共制造了469000辆，而在1929年仅trucks单独就有原来总数两倍那么多从组装线上下线。

考点：
1． twice只能作副词, 在句子中表达的意思却是比较, 这样的题目, 首先要弄清楚比较的双方是否合理, 如果合理的话, 再看twice作为副词的位置:
可以用在as..as或比较级之前, 也可以放在比较点的名词之 前, 担要保证twice前后的词要能直接搭配(副词的作用).
eg: He is twice the man he was. = He has twice the strength he had =He is twice as strong as he was.
2． Number的用法： a number of（大量的）， the number of（……的数量）， numbers of （许多）。

选项分析：
A. the numbers错误， twice后面要跟单数的number，即twice the number；但是改为the number后，指代特定的数字还是用that number更为明确
B. Correct； that number指代前面总和的469000，如果是the number，没有指代对象，似乎说是the number of trucks
C. the number指代不清； by themselves是指卡车出售时没有附带配件，即单车出售，与alone意思并不相 同。
D. 用as many trucks时，整个句子的意思是“1914年汽车和卡车一共生产了469,000辆，而1929年两倍的车下了生产线”，句子前后并没有直接逻辑联系，前面句子并没有讲卡车有多少辆；
trucks themselves 这样的结构（X itself）中， themselves反身代词是对trucks的同位语，表示强调是trucks而已，我们往往可以看作没有反身代词，就如同同位语往往与句子主成份无关一样。
E. as many trucks错误同D； by themselves错误同C

From Ron:
two problems with (d).
1. the meaning of the sentence is that the number of trucks in 1929 was almost twice 469,000.
choice (b) conveys this idea accurately, with the phrase "twice that number". see, "that number" must refer to an actual number cited in the sentence, and there is only one such number. mission accomplished.
choice (d) DOES NOT convey this idea. that choice says "twice AS MANY trucks", which means "twice as many trucks as were produced in 1914".
the problem is that the sentence doesn't tell us how many trucks were produced in 1914 - the only figure given is a combined figure for cars and trucks - so this statement doesn't make any sense in context.
not to mention, the intended meaning (from the original flawed sentence) is clearly that of (b).
2. "trucks themselves" doesn't make sense.
you don't use "x itself" unless you are trying to emphasize some element of the inherent nature of x (as opposed to something associated with x, or with some part of x).
example: pet accessories are becoming more and more popular, even though pets themselves have maintained constant popularity.
i.e., we want to emphasize that the second part of the sentence deals with pets themselves (as opposed to associated things such as pet accessories).
"trucks alone", though, makes perfect sense.

that number指前面的469000

卡车的数量既可以是 trucks' numbers 也可以是trucks‘ number
the numbers of trucks表示卡车的数量，没有说是什么时候的数量，也没用与前面提到的469,000扯上关系
alone类似于bu，作副词，表示仅仅，只。

错选a
没有the numbers of, 应该是the number of..注意看仔细
b正确

有the numbers of 这种说法吗？没有吧……

首先不应该用themselves，oneself表示某人对自己进行了一个动作，卡车自己不会come off lines，
A vs B，
A：the numbers of trucks核心词是number，in 1929 almost twice the numbers of trucks alone came off the assembly lines翻译过来就是1929 almost twice 469000 alone came off the assembly lines, 缺主语
B：that numbers of trucks核心词是trucks，that numbers指代469000，翻译过来就是1929 almost twice 469000 trucks alone came off the assembly lines
as many表示同样数量的，即469000，twice as many trucks是可以的，错的是themselves
alone可以做副词，修饰形容词twice，表示仅仅是它的两倍

B选项that number指代469,000，而D和E的as many trucks 指卡车的数量，原文没有说卡车有多少个。
by themselves" conveys the idea that the trucks came off the assembly line automatically by themselves

d.twice as many trunks指的是twice469000中的trucks...不合题意

注意B选项中的是that ！！！不是the

正确句意为：1914年cars和trucks共制造了469000辆，而在1929年仅trucks单独就有原来总数两倍那么多从组装线上下线。

A. the numbers错误，twice后面要跟单数的number，即twice the number；但是改为the number后，指代特定的数字还是用that number更为明确
B. Correct；that number指代前面总和的469000，如果是the number，没有指代对象，似乎说是the number of trucks
C. the number指代不清；by themselves是指卡车出售时没有附带配件，即单车出售，与alone意思并不相同。
D. 用as many trucks时，整个句子的意思是“1914年汽车和卡车一共生产了469,000辆，而1929年两倍的卡车下了生产线”，句子前后并没有直接逻辑联系，前面句子并没有讲卡车有多少辆；trucks themselves这样的结构（X itself）中，themselves反身代词是对trucks的同位语，表示强调是trucks而已，我们往往可以看作没有反身代词，就如同同位语往往与句子主成份无关一样。
E. as many trucks错误同D；by themselves错误同C

这真是Prep里面的题吗？感觉好怪

选项A：细节错误
the numbers

1914年美国总共生产了469,000辆轿车和卡车，但1929仅卡车就有2倍的产量下线。

选项C和D as many，前句并未给出具体truck的具体数字。