Diagonal Matrix

Q. If $D_1$ and $D_2$ are two $3\times 3$ diagonal matrices, then which of the following is/are true?

1. $D_1{D}_2$ is diagonal matrix
2. $D_1{D}_2=D_2{D}_1$
   
3. $D_1^2+D_2^2$ is a diagonal matrix
   
4. none of the above