Juz picked up a 28mm lens at abt 50% price from a clearance sales at a camera shop. Saw some marks on the rear element when I examined it at the shop but took them to be fingerprints and bought the lens. However upon bringing home and cleaning with a microfibre cloth, Found the marks to be quite impossible to remove. The are all along the rim of the rear element so shouldnt affect opical performance. Dun look like fungus to me but im not too sure. Should I

1) Ask for a refund if that is possible?

2) Keep but send to distributor if they can clean it?

3) Go for one of the 3rd party servicings for a second opinion although I got 1 yr warranty?