DC Corporation is analysing its fixed and variable costs within its current relevant range. As its activity changes within the relevant range, which of the following statements is$/$are correct?

I. As the activity level increases, the total fixed cost remains unchanged.

II$.$ As the activity level increases, unit fixed cost increases.

III. As the activity level decreases, unit variable cost decreases.

$$

A$.$

I, II$,$ and III are correct.

B$.$

I and II are correct.

C$.$

Only I is correct.

D$.$

II and III only are correct.