Carbonyl compounds: explain question

A student investigates an unknown liquid, X. The student carries out the following tests.

Test 1: A few drops of X are added to warm Fehling's solution. A red precipitate forms.

Test 2: A few drops of X are added to a solution of 2,4-dinitrophenylhydrazine reagent. An orange precipitate forms.

Test 3: X is warmed with alkaline aqueous iodine. A yellow precipitate forms.

Explain what each test tells the student about the structure of X, and deduce the structural feature that X must contain.