What Do the Door-In-The-Face Technique A

What do the door-in-the-face technique a.k.a. the rejection-then-retreat technique) and the foot-in-the-door technique have in common?

A) They both employ the principle of reciprocity.
B) They both operate through automatic, shortcut responding.
C) They both work only for door-to-door salespeople.
D) Both a and b