Otto used 6 cups of whole wheat flour and x cups of white flour in the recipe. which of the followings equation can be used to find the value of y, the total amount of flour that otto used in the recipe, and what are the constraints on the values of x and y?

a. y=6x; x is any integer greater than or equal to 0, and y is an integer greater than or equal to 6.
b. y=6x; x is any real number greater than or equal to 0, and y is any real number greater than or equal to 6.
c. y=x+6; x is any integer greater than or equal to 0, and y is an integer greater than or equal to 6.
d. y=x+6; x is any real number greater than or equal to 0, and y is any real number greater than or equal to 6.