a credit card / credit card(s)

[Itasan]

Do these all work?
1-1. Do you accept credit card?
1-2. Do you accept a credit card?
1-3. Do you accept credit cards?
2-1. I'll pay by credit card.
2-2. I'll pay by a credit card.
Thank you.

[Casiopea]

This one doesn't work:

1-1. Do you accept credit card?

This one is iffy. It depends on context:

1-2. Do you accept a credit card? (With emphasis on "a")

[Itasan]

Thank you very much, Casiopea.

[Casiopea]

You're welcome, Itasan.