If u = {all positive integers} and a = {x|x ∈ u and x is an odd positive integer}, which describes ac? ac = {x|x ∈ u and is a negative integer} ac = {x|x ∈ u and is zero} ac = {x|x ∈ u and is not an integer} ac = {x|x ∈ u and is an even positive integer}