B: benefit: 5000 
  Cost: 1000 (opp cost), he needs to buy first class ticket: 2000,
  Net benefit: 2000
 A: benefit: 5000
  Cost: if he buys first class ticket: 2000,
  if he buys second class ticekt: 1500, and the loss of unable to use computer on plane: 500,
  Net benefit: 3000
 C: benefit: 5000, and the opptunity benefit: 1000
  Cost: first class ticket: 2000
  Net benefit: 4000
 Choose C