The Sizes of Two Matrices Are Given $A$ Is $3 \times 1$

The sizes of two matrices are given. Find the size of the product AB and the size of the product BA, if the given product
can be calculated.
- $A$ is $3 \times 1$ ; B is $1 \times 3$ .

A) $\mathrm { AB }$ cannot be calculated; $\mathrm { BA }$ is $1 \times 1$ .
B) $\mathrm { AB }$ is $1 \times 1 ; \mathrm { BA }$ is $3 \times 3$ .
C) $\mathrm { AB }$ is $3 \times 3$ ; BA cannot be calculated.
D) $\mathrm { AB }$ is $3 \times 3$ ; $\mathrm { BA }$ is $1 \times 1$ .