Solve the following homogeneous linear ordinary differential equations. What is the dimension of the solution space? (a) $u^{\prime \prime}-4 u=0$, (b) $u^{\prime \prime}-6 u^{\prime}+8 u=0$,
(c) $u^{\prime \prime \prime}-9 u^{\prime}=0$,
(d) $u^{\prime \prime \prime \prime}+4 u^{\prime \prime \prime}-u^{\prime \prime}-16 u^{\prime}-12 u=0$.