Problem $1$: System Classification

Please check if the following systems are linear or nonlinear.

$($a$)$

$y(t)=2u(t)+t$

$($b$)$

$y^{}(t)=2u(3t)$

$($c$)$

$y^{}(t)-2y(t)=3u^2(t)$

$($d$)$

$y^{}(t)=sin(u(t))$

Show all steps of proving linear or non linear.