Give a reason why. You are not making excuses here—you’re providing a value reason why you’re unable to respond to an offer. Usually a statement like this works well: “I am working on the __ project right now and simply don’t have time to work with you on ___.” Make your decision quickly. Don’t waste time analyzing each opportunity. Like the quote from before, if something isn’t an automatic “hell yeah,” then your default decision is to say no.
