The clock was stopped after this play because there was a foul - that's the key here. It might also have been stopped after the play for other reasons (to award a first down to either A or B depending on the results of the play) but the situation never got that far. The clock was stopped because a flag was on the ground.
After the penalty is administered, the clock is started on the ready because the action that stopped the clock was to administer a penalty and the action that caused the down to end (A being tacked in bounds) would not have otherwise caused the clock to stop.
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