帮忙看到英语题，解释一下原因谢谢

Question

13. I was born ____.
A. in her sixties B. in early sixties
C. in the sixtieth D. in the sixties
14. He finished university in ____.
A. twenty B. twenties
C. early twenties D. his early twenties
15. The old man bought ____ eggs.
A. three score B. three scores
C. three scores of D. scores three
16. He wrote ____ article on computer.
A. a 800-word B. 800-words
C. 800-word D. an 800-word
17. The boy had ____ sheep to see to.
A. a lot of B. a large amount of
C. a great deal of D. a great many of
18. The police talked to him because he was ____ to enter the hall and the last to leave.
A. No. One B. one
C. the first D. first
19. Every year ____ students from this school are send to Britain to learn English.
A. six dozen B. six dozens
C. sixth dozen D. sixth dozen of
20. There’re 30 girls in our class,____ are from the country.
A. Three they B. The three of them
C. Three of them D. All three of them
谢谢各位主要是解释一下为什么选这个

Answer 1

13，D：固定用法，in the sixties，“六十年代”

14，D：“在他二十岁出头时”。在某人多少岁时的说法就是这样，twenties 前面要加上his/her/my等。

15，A：
数量词+SCORE+名词的时候不用OF
但是如果SCORE后面是代词比如THEM、THESE的话要用TWO SCORE OF THEM的形式
SCORES OF+名词 表示许多的含义了 此时SCORE要加“S”，其他时候不加

16，D：首先，像800-word这样中间加连字符的，那么后面那个名词就不能用复数，所以B是错的。第二，必须是wrote an article或worte the article，而不能只是worte article。最后，800的8发音是[e]原音开头，所以不定冠词必须是an，而不能是a。

17，A：四个答案中只有A是修饰可数名词，其余三个只能修饰不可数名词。而sheep是可数名词，只不过其单复数形式相同。

18，C：他是第一个。。。最高级应该加定冠词the，所以应该是 the first one to enter... 其中one可以省略。

19，A：把dozen当作hundred来理解，两者的用法一样。
用作数词（一打，12个）时，即使前面的数词大于1，也用单数，比如：six hundred students（600个学生）。表示很多个的时候才加S：hundreds of students（数百个学生）

20，C：可以说 three of them （其中三个），或是 all of them （他们全部），或是 none of them （他们一个都不）。没什么道理，就是这样用的。

Answer 2

a
d
a
d
a
c
a
c

Answer 3

DDADAAA,最后一题不确定，应该选C