West leads the ♥9 against your 3NT. What card do you play from dummy and why?
The bidding and the opening lead strongly suggest that East has over-called a suit headed by the ace–jack. If you play low from dummy, East will also play low. If you duck in your hand as well, West will play another heart, which East will win with the ace before playing a third round of the suit. If East has the ♦A too, he will win it the first time you attack the suit to finish running the hearts to defeat the contract.
Similarly, if you play the 10 from dummy, East will cover with the jack. If you decide to let that hold the trick, East will clear the heart suit and wait for the ♦A.
Can you do better by winning the queen at trick one? Not on this layout:
When West wins the ♦A, he’ll play another heart through dummy’s K–10 to beat 3NT.
So what’s the answer? Play the ♥K from dummy at trick one. Whether East wins or ducks this, you’re still in control. The hearts are safe from attack, so you’ll have the timing to establish the long diamonds in dummy, and it doesn’t matter which defender has the ♦A.