I know the correct answer is b, because that sounds correct.
I am trying to figure out why b is correct, and also why the other answers are incorrect. I think b is correct because ~ing signifies that it is happening now, and will continue into the future. c is obviously wrong because it is past tense. Also, if d did not include "ever", i.e. "having decreased", would that be correct?
What is the general grammar principle for "With..."?