$8.$If f$($n$)$ is big O$(2^$n$)$ and g$($n$)$ is big O$($n$^2),$ then which of these statements is true?

A$.$ f$($n$)$g$($n$)$ is bigO$(4^$n$)$

B$.$ f$($n$)+$ g$($n$)$ is big $)($n$^4)$

C$.$ f$($n$)+$g$($n$)$ is big O$(2$n$^2)$

D$.$ f$($n$)$g$($n$)$ is big O$($n$^4)$