Làm giải thích cho em tại sao dùng cái đó

Em đang cần gấp với

3. People perform a lion dance to bring (a/an/ the/0) good luck. 4. It's (a/an/ the/0) custom in my family to have dinner at home on Saturdays. 5. What's (a/an/ the/0) matter? Are you all right? 6. Kate has bought a new car since (a/an/ the/0) yesterday. 7. Some people think that (a/an ...