Unit 1: Bolivia's alternative crops; opprotunity cost and the PPC

This BBC article explains that Bolivia's farmers are moving out of growing coca (the basis for cocaine) and in to growing rice

http://news.bbc.co.uk/1/hi/business/7760261.stm

a) Explain the reasons why this change in crops has occured
b) Using a PPC, explain the opportunity cost of growing more rice