(A) If Dave is home, then his lights must be on.

(B) Dave's lights are not on.

(C) So, Dave must not be home.

(A) If Dave is at work, his car would be in the parking lot.

(B) Dave's car is not in the parking lot.

(C) Therefore, Dave must not be at work.

(A) Alice is a teacher.

(B) Since all teachers are friendly,

(C) Alice is friendly.

(A) Bob likes jam.

(B) Furthermore, Carol likes jam.

(C) So, Bob and Carol like the same thing.