The sysum $1$ is described by the input$-$output retationship $z(t)=t-1,$ and the system $2$ is described by the input$-$autput relationship $\frac{t}{6}y(t)+2ty(t)=x(t).$ Select the currect statement from the following choicies

Sulect one:

a$.$ Systerin $1$ and system $2$ are lineari.

b$.$ Sratem $1$ and instem $2$ are time imariant.

c$.$ System in nan$-$lineac, and system zas the yarying.