Consider the following statements for a system given by $$y\left( n \right) = x\left( n \right)\sum\nolimits_{k = - \infty }^\infty {\delta \left( {n - 3k} \right)} .$$
1. The system is linear.
2. The system is non-linear.
3. The system is causal.
4. The system is non-causal.
Which of the above statements is/are correct?

A. 1 only

B. 2 only

C. 2 and 4 only

D. 1 and 3 only

Answer: Option D