Which of the Following Helps to Explain Why the Supply

Which of the following helps to explain why the supply curve of labor is upward sloping?

A) The supply curve of labor is a derived supply curve; since the output supply curve is upward-sloping, so is the labor supply curve.
B) As the wage rate rises, the income effect causes the quantity of labor supplied to increase.
C) The substitution effect of a price change makes a good less expensive relative to other goods.
D) As the wage rate rises, the opportunity cost of leisure rises.