Can I use my Mastercard at Costco

I think not. Before now they had an agreement with American Express. Now the agreement is with Visa. Unless there is an exception for debit cards that I'm not aware of.

Yes, but might be limited to debit only. Check with your local store to find out about debit/credit use.

You can use Mastercard as debit only. The only cc they accept is Visa.

No

No. They only take visa my

Yes.

Of course

Ues you can.

I believe you can