Stationary processes...an easy way?

Hello again everyone,

I have a homework dealing with stationary processes. I'm hoping
someone can give me some hints to be at least able to detect what is
and what isn't a stationary process upon inspection of an equation,
given the parameters:

{Zsubt} = sequence of normal random variables with mean 0 and variance
sigma^2;
a,b,c = constants

Xsubt = Zsub1 cos(ct) + Zsub2 sin(ct).

My answer is that this *is* stationary because the cos & sin & (ct)
cancel eachother out? Is that an acceptable way of approaching this?

And, is there an easy way of finding out the mean and autocovariance
functions?

Thanks so much.

.