As I'm mid range C cat, I'd say you'd be B cat easily. I'd suggest start with B and if they do drop you, you'll at least get a draft/drag along for a free ride until the fast C's catch up
Most likely though you'll stay with the B's. Get yourself into a group and keep your nose out the wind. Drafting and tactics work so much more in Zwift racing.
If you get dropped from one group, but see another behind, don't try to hold them off. If there are 4-5 of them they will almost certainly catch you and for less effort than you'll put out trying to hold them off. Best to cruise along, have a rest, then when they get to you latch on to them. Then take them on the sprint finish