伝聞：そうだ vs ということだ


Here is a multiple-choice question from my grammar exercise book. I can rule
out all but two choices below.

> 医者の話では、2ヵ月ぐらいで退院できるだろう＿＿。(a.そうだ b.ということだ）

The correct answer according to the book is “b.ということだ”, but is “a.そうだ”
incorrect?

Edit: The relevant section of the book covers ということだ but not そうだ.




～だろう already indicates hearsay, so adding another layer of hearsay (そうだ) to it
makes it sound incredibly awkward.

The concept is similar to how you can't add the word "didn't" to verbs already
in the past tense (i.e. "didn't saw") in English.

You can however, conjoin そうだ to だ giving you something like this:

> 医者の話では、２ヶ月ぐらいで退院できるそうだ

