Why is the answer not D?

Hi, I'm not a tutor and I don't know if you still needed an answer on this one but I think it's because answer choice D weakens the argument by implying it wasn't the improved skill of the drivers that reduced fatalities, but instead it was the increased number of emergency hospital facilities. Essentially, it introduces an alternative cause for the observed effect (effect is reduced fatalities).

Thank you. That helped a lot!