In the short run:

A) all inputs are fixed.

B) all inputs are variable.

C) some inputs are fixed and some inputs are variable.

D) all costs are variable

C) some inputs are fixed and some inputs are variable.

Explanation:

Usually, the fixed cost are usually fixed for a some levels of activity. However, as the level of activities increases, the fixed cost may also increase.

Variable cost on the other hand changes directly as the level of activities (say number of units produced) changes.

As such, in the short run, some inputs are fixed and some inputs are variable.

Option C is right.