139- If $A = [a_{ij}]_{r \times 3}$ and $B = [b_{ij}]_{r \times 2}$, which of the following matrix products is defined? (1) $AB$ (2) $A^t B$ (3) $B^t A^t$ (4) $AB^t$
\textbf{139-} If $A = [a_{ij}]_{r \times 3}$ and $B = [b_{ij}]_{r \times 2}$, which of the following matrix products is defined?
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(1) $AB$ \hfill (2) $A^t B$ \hfill (3) $B^t A^t$ \hfill (4) $AB^t$
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