While testing for equality of variances, which of the following formulae gives the F-test statistic?
A) F = variance of sample 1 / variance of sample 2
B) F = variance of sample 1 + variance of sample 2
C) F = variance of sample 1 / variance of sample 2 * degrees of freedom
D) F = variance of sample 1 / variance of sample 2 * (sample size - 1)