Which of the following is correct when there is one input (labor) in the production?

a) when average product of labor equals to marginal product of labor, total product is at the maximum

b) when total product is rising, marginal product may be falling or rising but positive

c) when average product of labor is below exceeds marginal product of labor, average product of labor is rising

d) when average product of labor is below the marginal product of labor, marginal product of labor must be rising