welcome to the club.
Did you hear about Ron Purewall? He is one of the SC experts and one who scored 800 on the GMAT.
He permanently repeats to all test takers : DO NOT QUESTION CORRECT ANSWER CHOICE FROM OFFICIAL GUIDE, ASK YOURSELF WHY IT IS CORRECT AND WHY OTHER ANSWER CHOICES ARE WRONG.
You are commiting the same mistake as others.
Here two rates are compared: the rate of addition and the rate of loss.
Therefore asnwers talking about rates of loss are wrong, so A and C are out.
B is out - becuase it has not any clear antecedent.
E is out - becuase it has a nonsense meaning.
Only D conveys clearly the intendent meaning.
Follow me, if you find my explanations useful.
Audaces fortuna juvat!