Ali can buy a ball or a pen. He decides to buy the pen. The opportunity cost of buying the pen is ball.
Rahul can buy a 1 cd or a burger meal. He decides to buy 1 cd. The opportunity cost of buying 1 cd is burger meal
The government of china can either spend $2 million on roads or computer technology. It decides on spending on roads. The opportunity cost of the building the road is computer technology