Can someone give an example of when B would not be sufficient. If x + y is even, how can x-y not be an even integer?
In B, its given that x+y is even integer. But we don't know if individually x or y is integer or not.
r.g. x = 1.8 and y=0.2
=> x+y = 2.0, an even integer
and x-y = 1.6, not an integer
the product would also be not an integer.
Together from 1 and 2, we can say that (x - y)*(x + y) is even. Hence IMO C
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