Question:
For any two matrices $A$ and $B$,
A. $\mathrm{AB}=\mathrm{BA}$ is always true
B. $\mathrm{AB}=\mathrm{BA}$ is never true
C. sometimes $\mathrm{AB}=\mathrm{BA}$ and sometimes $\mathrm{AB} \neq \mathrm{BA}$
D. whenever $\mathrm{AB}$ exists, then $\mathrm{BA}$ exists
Solution:
