Oh, Fork!

Katie was playing black and sarah was playing white in this first round game. Katie has just moved her knight to B4 putting pressure on Sarahs white queen. Sarah plays queen to A3 which proves to be a fatal blunder. Can you see why and find Sarahs best move instead? 

By playing A4, Sarah allows herself to be forked on the next move by katie playing C2. A better move for sarah would have been B3. Post in the comments below if you know why the computer recognizes B3 as Sarahs best move.