Verify this problem with using BIg O

$($a$)$ f$($n$)=$O$($f$($n$))$

$($b$)$ if f$($n$)=$O$($g$($n$)),$ c$*$f$($n$)=$O$($g$($n$))$ is true $($c is random real number$)$

$($c$)$ if f$($n$)=$O$($g$()$n$)$ and g$($n$)=$O$($h$($n$)),$ f$($n$)=$O$($h$($n$))$