$4.$ In a single sentence each, carefully explain

$($a$)$ The conditions under which the Kutta Joukowski theorem can be used to calculate the force on a body in two$-$dimensional steady ideal flow.

$($b$)$ The reason that the Kutta condition is necessary for the ideal$-$flow analysis of airfoils

$($c$)$ The universal constraints on a vortex tube, regardless of flow type, according to Helmholtz theorems

$($d$)$ The conditions under which vorticity travels with the fluid material

$($e$)$ Stokes' hypothesis

$($f$)$ What a Newtonian fluid is