Better to ask the firm who gave you the offer for more time to consider it. Try to give them a reasonable estimate - they shouldn't be rushing you to sign a contract for work you won't do for another 11 months. You don't have to tell them you're applying elsewhere unless they ask.At the end of the day, if you aren't successful in your other applications you'll probably not be happy in a "second choice" job. Decide whether you really want to work there - if not and you're just holding on in case, then turn down the offer.If you think you will want to reapply to your first choice firms if you are unsuccessful this year, then you are far better to go travelling or volunteer or do something that isn't a grad job. That way, when you reapply you can say you were doing something developmental. If you apply from a second choice firm then a) you will look like you lack commitment and b) you will look desperate.