A. Xiao Li gave Xiao Wang Mai 100 yuan as a gift.
B. Xiao Li got a haircut a week ago 10 yuan.
C Xiao Li had lunch with 20 yuan money on his way to Xiao Wang's house.
D. Xiao Li gave up listening to philosophy lectures in order to attend the wedding.
Answer: b, c
Analysis: A is the opportunity cost, because using this 100 yuan to buy gifts gives up other possible uses. B it's not an opportunity cost, because the haircut is not for attending the wedding, but also for not attending the wedding. C is not an opportunity cost, because whether you attend a wedding or not, you have to have lunch. D is the opportunity cost, because it is a philosophy lecture given up for the wedding.
Economics is so bt. Many experts just like to complicate simple problems.