which one of the following statements related to variable costs is correct?

$-$ when production is halted, the variable costs of a project equal zero

$-$ variable costs fluctuate over the long term, but remain constant over the short term

$-$ the variable cost per unit is equal to the marginal cost at any level of output

$-$ variable costs change only when the quantity is varied by at least $5$ percent

$-$ an increase in the variable cost per unit increases the contribution margin