Also in C, we have
Math apps used to X, contributed to Y that <improve >
Shouldn't we have if C were correct ?
Math apps used to X, contributed to Y that <improved>
not if you're talking about drugs that improve patients' quality of life, such as the ones in this problem.
if you use the past tense, then it implies that whatever you're talking about is actually IN THE PAST. i.e., it's not actually happening anymore; whatever effect is under discussion is no longer occurring.
in this sentence, the most reasonable interpretation is that the drugs STILL contribute to quality of life, even though the research in the question has already been completed. thus the different tenses.
here's another example:in the 16th century, physicians discovered that human blood circulates through the body.
this is correct.
you don't want to say "circulated", since, of course, blood still circulates in the present world.