Find the domain and range of the relation and determine whether it is a function.
A. domain: all real numbers; range: all real numbers; Yes, it is a function.
B. domain: positive integers; range: positive integers; No, it is not a function.
C. domain: x ≥ 0; range: y > 3; No, it is not a function.
D. domain: x > 3; range: y > 0; Yes, it is a function.