Difference between ないからこそ and もいないんだから in practice question

Question

I haven't made sense of the answer to the following question.

試験の点が悪かったことを母に伝えると、「ちゃんと勉強( ) 、良い点をとろうなんて無理でしょう」と言われてしまった。

a) してはいけないのに

b) していないからこそ

c) してもいないんだから

d) しようとしていながら

The correct answer according to the book is c) however I can't figure out why it shouldn't be b) as both sound identical to me in English (Every time I tell my mom about bad grades on an exam, she says: "If you don't even study, of course, getting good grades is impossible!")

What would be the difference between b) and c)? The book only explains why c) is the answer without any context on the rest.

Answer 1

The whole sentence becomes weird with the added こそ. The nuance between the first part of the sentence doesn't match with second part. からこそ is more commonly used to explain a positive reason:

誰もこのコースに出願していないからこそ、あなたは今すぐに入るべきだ。

Nobody has applied for the course yet so that's why you should be able to enter right away.

In your question/example, してもいないんだから is matching with your translation. "She said, If you don't even study seriously, of course getting good grades is impossible."