Problem $1$: Order of PDE

Find the order of each of the following PDEs

a$)$ The convection or advection equation $u_t+c{u}_x=0.$

b$)$ The wave equation $u_{tt}=c^2{u}_{}.$

c$)$ The eikonal equation $u_x^2+u_y^2=1.$

d$)$ The Euler$-$Bernoulli beam equation $u_{tt}+{}^4{u}_{}=0$