What Makes World's Fair of 1876, in Philadelphia a Natural

Multiple Choice

What makes World's Fair of 1876, in Philadelphia a natural experiment suitable to study the impact of patent protection on innovation? (The author of the study also used the data from the 1851 Fair in London.)

A) Many countries with similar patent laws were represented in the fair, but it was difficult to hold a patent outside the country of origin.
B) Many countries with varying patent laws were represented in the fair, and it was difficult to hold a patent outside the country of origin.
C) Many different types of innovations were exhibited in the fair from countries with similar patent laws.
D) Many innovations of same types were exhibited in the fair from countries with varying patent laws.