A shipping company offers a $$20,000$ reward to the first business to locate one of the company's ships that sunk off the coast of Nova Scotia, laden with valuable cargo. This is an example of

a$.$ A unilateral contract that can be accepted by performance

b$.$ A unilateral contract that will be accepted by a business that promises to conduct the search

c$.$ A bilateral contract that can be accepted only by performance

d$.$ An invitation to treat