Identify each relation that is also a function. For each relation that is not a function, explain

Question:

Identify each relation that is also a function. For each relation that is not a function, explain why not.

a. independent variable: city dependent variable: area code.
b. independent variable: any pair of whole numbers dependent variable: their greatest common factor.
c. independent variable: any pair of fractions dependent variable: their common denominator.
d. independent variable: the day of the year dependent variable: the time of sunrise