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If an institution tells you, look here is an admission offer, and you should let’s know if you accept it or reject it by the day after tomorrow. Meanwhile, you are still waiting for a word from other places that you applied to.
So, what is the most diplomatic and tactful way to communicate “Hey, I can not accept/reject the current offer right now. That's because i am waiting to hear back from other schools, and I need time to decide upon that”? At the same time, I do not want to sound like careless about the offer.