Fuente: ACBL
Test your Play
Solutions:
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
⇓
Solutions:
1. 
Win the 
A, cross to the 
Q, play the 
A and ruff a diamond. Next, play two high trumps, discarding clubs. Now over to the 
K unblocking your jack, and ruff another diamond. If both follow to three rounds of diamonds, you have the dummy entries to set up dummy’s fifth diamond for your 13th trick.
If diamonds are 5-2 and West has five diamonds, cross to the K, ruff another diamond and play your last trump. This will be the three-card ending when you are about to cash your last spade: 
Play your last spade: If West started with a 2=2=5=4 pattern, he will be down to two clubs and a diamond and will be forced to discard a club as you discard dummy’s diamond. When you lead a club and West follows, you know his other card is a diamond so you will rise with the ace and will catch East’s now bare queen. A count squeeze.
In addition, if West started with a 3=2=5=3 pattern, East, having started with six hearts, is the only one guarding hearts. West will be down to two clubs and a diamond and East will be down to two clubs and a heart. West must discard a club on the last spade, dummy discards a diamond, and East must discard a club to keep the master heart. Both defenders are down to one club, so the A will scoop up the two missing clubs and the 10 will be your 13th trick. A double squeeze.
If East has five diamonds, take the club finesse. It is too risky to try to develop clubs before drawing trumps. If clubs are 4-2 and West has a doubleton, there is a huge danger of an over-ruff. If East has a doubleton, there is a danger of an uppercut on the third club if East holds 10–x or J–x of spades. And if you try to develop clubs after your draw trumps, there will be only one side entry to dummy, and 4-2 clubs could do you in. Diamonds before clubs.
This is a trick-one problem. From the get go, you know that East has the A, the K (maybe the J), and at least the K, probably K J x (x). So how are you going to make this contract assuming the K is not singleton?
If West has led from Q-x or the Q–J–x of clubs, there is no way to make it. You are destined to lose two clubs, the and a diamond. If West started with the singleton Q and you duck, you are at the mercy of a spade shift followed by a high club which West will ruff. That’s three losers, and even though a losing club from dummy can go on a spade, you are still down at least one.
Your legitimate play to land this contract is to hope West started with the Q–J doubleton. So win the A while the suit is blocked, draw trumps and knock out the A. The best East can do is win the A and lead a club to West’s jack.
West now has a choice of two exit suits, spades or diamonds. If West exits a spade, discard a diamond from dummy on the third spade and exit a club to East’s king. East must lead away from the K or give you a ruff and a sluff. Either play allows you to avoid a diamond loser and make the contract. If West gets out with a low diamond, insert the 8 and hope West has led from the 10 or jack. After East covers the 8 with the 10 or jack, win the ace, discard dummy’s remaining club on a third spade and lead a diamond to the 9. As long as East wasn’t born with the K–J-10 of diamonds, you live to fight another day — but you had to win the A at trick one to stay alive!