Following the discussion from the lessons, which of the foilowing are valid examples of class invariants? Select one or more:

a) In the array-based stack implementation, if the stack is not empty, the topmost item should be at position stack_size 1 in the values array, and all the values before that are below it on the stack

b) In the array-based queue implementation, the user should not call push when the queue is empty

c) In the rational number class, the denominator should be greater than 0, and the numerator and denominator should not have a GCD greater than 1

d) in a clock class, the hours field should be between 0 and 23 (inclusive), and the minutes and seconds between 0 and 59 (inclusive)

e) In the rational number class, the user should not try to construct a rational with a denominator of 0

f) In the car class, the user should not call the turn_off function when the speed is greater than 0