15. A rectangle with sides ( $2 n - 1$ ) and ( $2 m - 1$ ) is divided into squares of unit length. The number of rectangle which can be formed with sides of odd length is :\\
(a) $m ^ { 2 } n ^ { 2 }$\\
(b) $m n ( m + 1 ) ( n + 1 )$\\
(c) $4 ^ { m + n - 1 }$\\
(d) none of these\\