Identify the relation that is not a function

a. (d,2) (d,-1) (1,-3)
b. (-2,d) (0,d) (1,-2)
c. (-a,5) (a,1) (2,0)
d. (a,2) (b,2) (c,2)

Answer is A because to be a function, it has to pass the vertical line test (draw a up and down line to find how many values are in each x). If there are more than one value of y in each x then it isn't a function. the line x = d has 2 points on it therefore it isn't a function

itz a. because...you see in a function each element in the domain must have a unique codomain...the same element cannot be matched with two different element in codomain...but here in 'a' d has two codomain ie 2 and -1...whereas in all others each element of domain has a unique codomain...so a is the answer cz its a relation but not a function