Which of the following operators are linear? Assume u $=$ u$($x$,$ y$,$ z$).($Problem $2\mathrm{.}3$B$)$ def $($a$)$u$=$u$($Laplacian$)$ def $($b$)$ L$[$u$]=$u$$xux def $2($c$)$ L$[$u$]=($u$)$ def $2($d$)$ L$[$u$]=$u def $($e$)$ L$[$u$]=$ uux$+$u def $($f$)$ L$[$u$]=$ ux$+$u$5$