Unless you were literally bored with the pace or it was slower than your average solo ride, consider taking a few more D rides before moving up. As you advance not only does the pace get faster but the expectations of the riders get higher too, drafting, signaling, establishing a paceline and taking a turn on the front. I don't want those expectations to rub you the wrong way. If C and D groups leave at the same place/time get there early and see if you can chat with the ride leader.

Good luck!