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.