23. The differential equation
$\frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2} = u$
is (Select one answer only)
(a) first order, linear, homogeneous
(b) first order, linear, non-homogeneous
(c) second order, nonlinear
(d) second order, linear, homogeneous
(e) second order, linear, non-homogeneous