Opportunity cost: It means that the same economic entity faces multiple decisions when engaging in an economic activity. If the selected decision is not the most valuable, then the difference between the most valuable decision and the selected decision is the opportunity cost. Based on the above definition, which of the following is the following understanding of opportunity cost? ['Xiao Zhao will get 50 yuan if he sells the old book to the newspaper collector, but he sells the book to the dormitory administrator for 20 yuan. He paid the opportunity cost of 30 yuan.', 'Xiaozhuang has 10,000 yuan in spare money. He invested in stocks and lost 2,000 yuan. If he invested in futures, he would gain 1,000 yuan. For this, he paid an opportunity cost of 8,000 yuan.', 'Xiao Jia, Xiao Zheng will receive an annual salary of 50,000 if he works in Company A; he will receive an annual salary of 40,000 if he works in Company B; he will receive an annual salary of 30,000 if he works in Company C. As a result, Xiao Jia chooses B Company, Xiao Zheng chose Company C, and they paid the same opportunity cost.', 'If Xiao Wang sells the car to Xiao Li, he will get 120,000 yuan; if he sells the car to Xiao Ming, he will get 100,000 yuan. As a result, he sold the car to Xiao Li, and he received an opportunity cost of 20,000 yuan.']