4. Databases (20分) You have the relations A, B and C shown as follows: A B C A1 A2 A3 B1 B2 C1 C2 C3 1 12 100 22 214 31 401 1006 2 16 102 24 216 32 401 1025 3 16 103 27 284 33 405 1065 4 19 104 29 216 (a) What is a relation in a relational database?