Tough situation, but if you already accepted the offer and signed the contract, I'd stick with A. It's a small world out there, and I hate burning bridges. If 60k is a senior dev salary, as Bee says, it'll be a more demanding position, and if this is your first job, you'll have a ton to learn already about being an employee and getting up-to-date on stuff--especially if you don't know C# already.
And finally, an internship is a summer job. Could you tell B you already accepted an internship, but you'd like to start after the summer?