PR WB-9-20

(A)(D)は、「approaching that」のthatもmarriage rates を指すのではないでしょうか？だとすると、単数形の(A)(D)は×で、
thoseを使っている(E)でOKなのだと思います。

so much so that がナンだ?という感じがしましたが、「〜するほどまで」、という意味だそうです。
-----
PR WB-9-20
(While marriage rates have fallen sharply in the last
decade, they are now approaching that in) the first four decades of this century.
A) While marriage rates have fallen sharply in the last
decade, they are now approaching that in
B) Because marriage rates have fallen sharply in the last
decade, it is now approaching those in
C) In that marriage rates have fallen sharply in the last
decade, it is now approaching those of
D) Marriage rates have fallen sharply in the last decade,
to the extent that they are now approaching that in
E) Marriage rates have fallen sharply in the last decade,
so much so that they are now approaching those of
正解E
ratesに対応するtheyを取ってADEまで選べたのですが、ADを切るポイントが分からず、また、Eのso much so thatがしっくりこないです。