Which of the following are true? Assume that x and y are real numbers.
(a) For every x, x > 0 ⇒ x2 > 0.
(b) For every x, x > 0 ⇔ x2 > 0.
(c) For every x, there exist a u such that y > x2.
(d) For every positive number y, there exists another positive number x such that 0 < x < y.