Which sentence uses the semicolon correctly?

1. ক
   I have a big test tomorrow; I can't go out tonight.
   
2. খ
   I have a big test tomorrow, I can't go out tonight.
   
3. গ
   I have a big test tomorrow I can't go out tonight.
   
4. ঘ
   I have a big test tomorrow: I can't go out tonight.