Which of the following is true of a constant cost production - possibilit y curve? a. A constant cost production possibilities curve is drawn as a positively sloped straight line. b. Along a constant cost production possibilities curve , the opportunity cost of producing more of a good is constant. c. When a country engages in free trade, the constant cost production - possibilit y curve shift s to the right. d. A country with a c onstant cost production - possibilit y curve partially specializ es in the production of goods when it engages in free trade with other nations .