Jian's weekly production possibilities frontier for doing homework (writing papers and doing problem sets). What is the opportunity cost of doing one problem set? [a.] The opportunity cost of doing one problem set is 1/2 paper. [b.] The opportunity cost of doing one problem set is 1/3 paper. [c.] The opportunity cost of doing one problem set is 1/4 paper. [d.] The opportunity cost of doing one problem set is 1/6 paper