GMAT Critical Reasoning (CR) | Re: The faster a car is traveling, the less time the driver has

I am still calling into question the reasoning behind answers B and D. As for D: the stem has already told us the fatality/mile rate dropped - so it is given, there ARE factors that either impact the numerator or denominator or both - D completes the picture/explains by saying that the denominator increased. Here we are sort of forced to make a slight assumption that the fatality number should have risen at most at a lower rate than the milage driven for the equation to hold true. BUT the stem