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