Which one of the following systems is a causal system? [y(t) is output and u(t) is an input step function]

y(t) = sin (u(t – 3)) + sin (u(t + 3))

Which one of the following systems is a causal system? [y(t) is output

1.	Which one of the following systems is a causal system? [y(t) is output and u(t) is an input step function]
A.	y(t) = sin (u(t + 3))
B.	y(t) = 5u(t) + 3u(t – 1)
C.	y(t) = 5u(t) + 3u(t + 1)
D.	y(t) = sin (u(t – 3)) + sin (u(t + 3))
Answer» C. y(t) = 5u(t) + 3u(t + 1)