Match each of the following differential equations with a solution from the list below.

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

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