What is the meaning, as used in economic analysis, of the statement, $200 is the present worth of $400 in ten years? $200:

a. Will buy now what $400 will buy in ten years b. Invested now will give a return in interest of $400 over a ten year period c. Invested now will give a return in interest plus $400 over a ten year period d. Compounded at a certain interest rate will become $400 in ten years