Answer:
B. for many firms, marginal and average variable costs are constant over wide ranges of output.
Step-by-step explanation:
Traditional Cost Theory : Marginal & Average Variable Cost are U shaped.
Modern theory of cost behaviour for real world firms suggest - 'short run average variable cost (SAVC)' is saucer shape curve, ie flat (constant) stretch over a wide range of output.
Such shape of SAVC is due to 'reserve capacity' of production by firms, to meet up unexpected demand change due to seasonal or consumer taste changes. This reserve capacity prevents the SAVC to rise immediately after falling (as per U shape) & rather sustains it constant for a wide range of output (as a saucer shape)