Match each of the given differential equations with one or more of these solutions:

(a) y = 0, 

(b) y = 2, 

(c) y = 2x, 

(d) y = 2x².

1. xy' = 2y

2. y' = 2

3. y' = 2y  4

4. xy' = y

5. y'' + 9y = 18

6. xy''  y' = 0