请教一道GWD上的题~~~

In 1914 a total of 469,000 cars and trucks were produced in the United States, but in 1929 almost twice the numbers of tucks alone came off the assembly lines.

A.    the numbers of tucks alone

B.    that number of trucks alone

C.    the number of trucks by themselves

D.    as many trucks themselves

E.     as many trucks by themselves

GWD TN24里一题，答案给的b.  没有怎么明白意思，请NN们指点下~~~

谢谢啦~~

1.twice +as many/much as

2.twice + n.~~~这里number 名词形式不用复数