Scoring: MP
South opens 2NT (19+ to 21) and is raised to 3NT. Your opening lead was a fourth-best club, ducked by dummy to partner's King. Partner swtched to the heart 10, won by your Ace when Declarer played the King. A heart back yielded a pitch by partner and Declarer won. Declarer played two more rounds in hearts, ending in dummy. The diamond Queen was played next, winning. Declarer next plays the spade Jack from dummy and then rises with the Ace, to you.
What spade do you play, and why?