TRICK 2.--B’s return of the three of hearts shows the situation to
A. It is the rule at “no-trump” to return partner’s suit with your
highest card. A can mark Z with two cards in the suit higher than the
three.
TRICK 3.--It would be bad play for A to lead his king of hearts, as
this would establish the queen for Z. A opens the low diamond, hoping
that B can obtain the lead.