In 1914 a total of 469,000 cars and trucks were produced in the United States , but in 1929 almost twice the numbers of trucks alone came off the assembly lines. A.the numbers of trucks alone
B.that number of trucks alone
C.the number of trucks by themselves
D.as many trucks themselves(B)
E.as many trucks by themselves
对答案没啥疑问,但是of trucks 是什么意思呢,为什么不是 that number by trucks alone !!!!