The covariance function $$C_x (\tau)$$ of a stationary stochastic process x(t) is said to be positive definite. This means that

A

$$C_x(\tau) \geq 0 for all \tau$$

B

$$\int_{-\infty}^{\infty}C_x(\tau) d\tau \geq 0 $$

C

$$\int_{-\infty}^{\infty}C_x(\tau) exp(-j \omega \tau) d \tau \geq 0 $$

D

$$C_x(0) \geq 0 $$