The systems given below have input x and output y $.$Determine whether each of them is memorlyless, stable, causal, linear and time invariant.

i$.,$y$($t$)=$x$(-$t$)$

ii$.,$y$($t$)=$x$($t$2)$

iii. $,$x$($t$)=$cos$(2$t$)$u$($t$)$

iv$.,$y$[$n$]=$nx$[$n$]$