26. A. each B. many C. much D. every 27. A. however B. therefore C. although D. otherwise 28. A. protection B. guarantee C. quality D. diversity 29. A. buyers B. customers C. guests D. vendors 30. A....

26B. many - The sentence is talking about a group of students, so “many” is the appropriate choice.
27A. however - This word introduces a contrast or contradiction to the previous statement.
28B. guarantee - In the context of shopping, especially in flea markets, a “guarantee” is what you lack when you don’t get a receipt.
29D. vendors - The ones who sell items and might refuse to give a refund are “vendors”.
30C. which - This refers to the specific item that you buy.