1. Average cost curves have the same shape as:
   1. Total cost curves
   2. Marginal cost curves
   3. Average fixed cost curves
   4. Total fixed cost curves
2. Where marginal cost is less than average cost,:
   1. Marginal cost must be falling
   2. Opportunity cost must have been excluded from the calculation of marginal cost
   3. Marginal cost may be rising, falling, or constant
   4. Marginal cost must be rising