because he doesn't leave immediately, I call a policeman.
A. Unless he leaves immediately, I won't call a policeman.
B. If he leaves immediately, I won't call a policeman.
C. Unless he leaves immediately, I will call a policeman.
D. If he left immediately, I wouldn't call a policeman.
3. Today isn't Sunday, so the pupils can't go swimming.
A. If today is Sunday, the pupils could go swimming.
B. If today were Sunday, the pupils could go swimming.
C. The pupils could go swimming unless today is Sunday.
D. The pupils could not go swimming if today isn't Sunday.
4. Stop talking or you won't understand the lesson.
A. If you don't stop talking, you won't understand the lesson.
B. If you don't stop talking, you wouldn't understand the lesson.
C. If you hadn't stopped talking, you wouldn't understand the lesson.
D. If you hadn't stopped talking, you wouldn't have understood the lesson.