The Power of Reciprocity, Even Among Those Who Claim to Be

The power of reciprocity, even among those who claim to be unaffected by it, is demonstrated by

A) differential support found by researchers regarding the safety of drugs depending on whether pharmaceutical companies were a source of researchers' funding.
B) the higher likelihood of US congressional representatives who received the most funding from special interest groups to vote in favor of the group who donated the most money to their campaign.
C) the difficulty that former President Jimmy Carter, a Washington-outsider, had in getting his programs through Congress
D) All of the above.