First Answer
You are on lead against three notrump.
You lead six of clubs (four, ten, Ace). Who has the King of clubs? Declarer has the King of clubs; partner would have played the King if partner had it. Declarer also has the nine (partner would have played the nine instead, from ten-nine).
Now declarer plays the Ace-King-Jack spades.
What will you discard to this trick: a heart, a diamond, or a club?