15. Let the predicate knows(x, y) again denote that person x knows person y, and happy(x) to mean that person x is happy. Which one of the following first order logic formulas encodes the claim "Every...

Extracted text: 15. Let the predicate knows(x, y) again denote that person x knows person y, and happy(x) to mean that person x is happy. Which one of the following first order logic formulas encodes the claim "Every happy person knows somebody who is not happy"? (a) V x : happy(x)= 3 y:( knows(x, y) = happy(y)) (b) x: у: (hаppу(x) Л kпows(х, у)) %— -һарру(у) (с) Э х: у:happy(x, у) 3 knows(x, y) (d) V x: 3 y:-happy(x) V ( knows(x, y) ^ -happy(y))