Professor Leeds asked his students to sign a petition for community center. Later, he asked the students who signed the petition if they would come…

Professor Leeds asked his students to sign a petition for community center. Later, he asked the students who signed the petition if they would come to the center and tutor a child. Professor Leeds has just used what technique to get the students to help?

A: A door in the face

B: Mere exposure

C: Lowballing

D: Foot in the door