Student or Learner
If you behave well from now on, you ( ) have a computer for your 18th birthday.
A will B shall C can D must
The given answer is B. I would have chosen A. We say 'I promise that you will have...', don't we? And 'you shall' sounds out-dated, doesn't it?
Would native speakers have chosen A or B?
Thank you in advance.
Like you. I'd have chosen A, though (possibly) I'd have scanned through the rest of the paper and realized that the examiner was after answers that reflected out-dated prescriptions (in which case I'd have held my nose and chosen B! [If you 'hold your nose' while you do something, you do that thing although you know it's wrong; the image derives from the practice of getting a child to hold its nose while drinking unpleasant medicine.])
The Fairy Godmother in Cinderella said 'You shall go to the ball', but we are in the real world now.