Why is the F-distribution always positive?

In the previous exercise, it was clearly visible that the F-distribution is not symetrical about the y-axis the same way the t- and z-distributions are. It is only defined for positive values of x. Which of the following statements is true?

Possible Answers

The F-distribution can take negative values as well; we just didn't consider this in the previous exercise.
The F-distribution cannot take negative values, because it represents the variance between groups and variances tend to be non-negative numbers.
The F-distribution cannot take negative values, because it is a ratio of variances and variances are always non-negative numbers. The distribution represents the ratio between the variance between groups and the variance within groups.

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Exercise

Why is the F-distribution always positive?

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