Consider the Following Three Functions

Consider the following three functions:
F(x) = 6x²
G(x, y) =4x - 3y + 19
H(x, y) = 3xy
Which of the following is true regarding the linearity of the functions?

A) none of the functions is linear
B) all three functions are linear
C) g(x, y) is linear, but f(x) and h(x, y) are not linear
D) g(x, y) and h(x, y) are linear, but f(x) is not linear
E) h(x, y) is linear, but f(x) and g(x, y) are not linear