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a.A normal distribution is any distribution that is not unusual.
This is not necessarily true; not unusual does not directly mean a "normal distribution".
b.The graph of a normal distribution is bell-shaped.
By definition, this is true.
c.If a population has a normal distribution, the mean and the median are not equal.
This may or may not be true; mean and median are different measures of central tendancy.
d.The graph of a normal distribution is symmetric.
Again, by definition, this is true.