Pizza Puzzle

When three monsters, Choral, Lavonda and DuWeine, eat at the Monster Snack Bar, each orders either pizza or their famous Monster jello salad.

If Choral orders pizza, Lavonda orders jello.

Either Choral or DuWeine orders pizza, but not both.

Lavonda and DuWeine do not both order jello.

HINT: Determine the monsters whose menu selections never changes.

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Solution:

From [1] and [2], if Choral orders pizza, Lavonda orders jello and DuWeine orders jello. This situation contradicts [3]. So Choral orders only jello.
Then, from [2], DuWeine orders only pizza.
So only Lavonda could have ordered pizza yesterday, jello today.