请教大全No.41---这道题好难！

这道题的正确答案是A,但是我真的不明白（1）为何用完成时态呢？难道是因为by? (2)A中those是否会有指代上的错误？those究竟是指代american还是one percent of american?还有B中they , C中those,我都没有想明白。总之这题好难，忘各位NN指教！！！谢谢！！！！

41．According to a study published by Dr. Myrna Weissman, only one percent of Americans born before 1905 had suffered major depression by the age of seventy-five; of those born since 1955, six percent had become depressed by age twenty-four.

(A) only one percent of Americans born before 1905 had suffered major depression by the age of seventy-five; of those born since 1955, six percent had become depressed by age twenty-four

(B) only one percent of Americans born before 1905 suffer major depression by the age of seventy-five; if they are born since 1955, six percent become depressed by age twenty-four

(C) of Americans born before 1905, only one percent of them have suffered major depression by age seventy-five, but six percent of those born since 1955 do by the age of twenty-four

(D) major depression is suffered by the age of seventy-five by only one percent of Americans born before 1905, and by age twenty-four by the six percent born since 1955

        (E) Americans born before 1905 suffer major depression by the age of seventy-five only one percent of the time, but six percent of those born since 1955 did so by age twenty-four

of those born since 1955, six percent had become depressed by age twenty-four

这里用完成时很恰当阿，看来楼主的复习复习时态了，因为一半句讲不清楚。

A中的they指代Americans。

by the age of。。。直到....是要用完成时态，还有这题目不难，用排除法马上选出答案为A了。

B.if they are born since 1955,罗嗦，而且与前面不平行对应

C.of Americans born......., only one percent of them ..........， but six percent of those ........有问题这种表达，非常不简洁，就是OG常说的awkward

D.major depression is suffered by the age of seventy-five ....哪里可以这样用被动表达

E.only one percent of the time, but six percent of those。。。。。非常不顺

其实很简单,根据时态,你就可以排除B,C,D,E.

A中those指代Americans.