Advice for how to respond to job offer

I've received a verbal offer from Company A which is my second choice. They have asked me to accept or decline the offer and I must do that before I receive the written offer according to their hiring policy. I also have a verbal offer from Company B which is my first choice and HR is processing the written letter which I am told will take a few weeks. I don't want to decline Company A right away in case Company B doesn't work out. I am also not sure exactly when I will get the written letter for Company B. How should I proceed?

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