Based on the information given, the correct options will be:
a.) Mary would use the sample standard deviation to calculate a t-statistic.
c.) The value for the degrees of freedom for Mary's sample population is six.
One-sample t-test simply means a statistical hypothesis test that's used for the determination of whether an unknown population mean will be different from the specific value.
In this case, the value of the degree of freedom for the sample will be:
= (n - 1) = 7 - 1 = 6.
Also, it should be noted that Mary would use the sample standard deviation to calculate a t-statistic.
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