Consider u = {x|x is a positive integer greater than 1}.

which is an empty set?

{x|x ∈ u andone-half is prime}
{x|x ∈ u and 2x is prime}
{x|x ∈ u andone-half can be written as a fraction}
{x|x ∈ u and 2x can be written as a fraction}