Consider u = {x|x is a positive integer greater than 1}. which is an empty set? {x|x ∈ u and 1/2x is prime} {x|x ∈ u and 2x is prime} {x|x ∈ u and 1/2x can be written as a fraction} {x|x ∈ u and 2x can be written as a fraction}