Find the solution of the differential equation \mathbf{r}'(t)=3 \mathbf{r}(t) with the initial condition \mathbf{r}(0)=\langle 9,10,9\rangle , where \mathbf{r}(t) is a vector-valued function in three-space. What is the solution of the differential equation?

A) \mathbf{r}(t)=\langle 9e^{3t},10e^{3t},9e^{3t}\rangle
B) \mathbf{r}(t)=\langle 9e^{3t},10e^{3t},9\rangle
C) \mathbf{r}(t)=\langle 9,10,9e^{3t}\rangle
D) \mathbf{r}(t)=\langle 9e^{3t},10,9e^{3t}\rangle