In a certain card game, one of the hands dealt contains:

1. Exactly thirteen cards.
2. At least one card in each suit.
3. A different number of cards in each suit.
4. A total of five hearts and diamonds.
5. A total of six hearts and spades.
6. Exactly two cards in the "trump" suit.

Which one of the four suits -- hearts, spades, diamonds, or clubs -- is the "trump" suit?