A system accepts a sequence of real numbers $x[n]$ as input and outputs
\[
y[n]=\left\{\begin{array}{ll}
0.5 x[n]-0.25 x[n-1], & n \text { even } \\
0.75 x[n], & n \text { odd }
\end{array}\right.
\]
The system is
- non-linear.
- non-causal.
- time-invariant.
- All of the above.
- None of the above.