The law of diminishing returns shows that, in the short run, _____.

as the marginal product of labour falls, variable costs rise
changes in productivity do not affect changes in costs
productivity changes in proportion to changes in fixed costs
productivity and costs change at the same rate

Explanation
Variable costs rise slowly initially; then, as output increases, they begin to rise more quickly. This simply reflects the law of diminishing returns. As additional workers become less productive, costs rise quicker than output.