Studies of the Effect of City Size on Helping Have

Studies of the effect of city size on helping have been conducted across the United States and in several countries around the world. They have found that

A) there is no association between city size and helping in any of the locations studied
B) there is an association between city size and helping, but only within the United States
C) there is an association between city size and helping, but only outside the United States
D) there is an association between city size and helping in every location studied