An airplane ascends at the rate of 40 feet per minute. The function below shows the height of the airplane from the ground f(t) after t minutes. f(t) = 40t + 520
Which statement best describes the y intercept of the graph of the function?

A It is equal to 520 and represents the height of the airplane above the ground, in feet, after 1 minute.
B It is equal to 560 and represents the height of the airplane above the ground, in feet, after 1 minute.
C It is equal to 40 and represents the height of the airplane above the ground, in feet, before it starts ascending.
D It is equal to 520 and represents the height of the airplane above the ground, in feet, before it starts ascending.