Match each of the differential equations with its solution.

A. $y = 3x + x^2$
B. $y = e^{-6 x}$
C. $y = x^{\frac{1}{2}}$
D. $y = \sin(x)$

1. $xy' -y = x^2$
2. $2x^2y'' + 3xy' = y$
3. $y'' + y = 0$
4. $y'' +9 y' + 18 y = 0$