Final answer:
It is true that in the short run, capital is fixed and only labor is a variable cost. Production volume can only be adjusted through changes in labor due to the nature of short-term capital constraints.
Step-by-step explanation:
In the context of production and costs, it is true that over the short run capital tends to be fixed and thus labor becomes the only variable cost of production. In the short term, production is typically represented by the function Q = f[L], reflecting that output (Q) varies as a function of labor (L) because capital (K) is fixed. The marginal product of labor illustrates the additional output from an extra unit of labor, which eventually decreases due to diminishing marginal productivity.