Answer
(a)
Among $m$ and $n$ only one must be negative.
(b)
Among $m$ and $n$ one of them must zero.
(c)
Among $m$ and $n$, either they both must be positive or they both must be negative.
Work Step by Step
(a)
We get a positive answer if we have even negative signs.
Since we already have one negative sign.
Among $m$ and $n$ only one must be negative.
(b)
If we multiply zero in any real number we get zero.
So, among $m$ and $n$ one of them must zero.
(c)
We get a negative answer if we have odd negative signs.
Since we already have one negative sign.
Among $m$ and $n$, either they both must be positive or they both must be negative.
