The purpose of the Open Door Policy was A. Open immigration for people who were imperialized
B. equal trade for all nations in China
C. when Africans welcomed Europeans to their countries
This rebe

B. Equal trade for all nations in China

the answer to fill in the blank in this question would be the word resource, it is defined as the money, material or people (since human resource is a very valuable good) that a company or a person has at disposition (meaning they can use the resource as they see fit), to be able to complete a task or achieve a goal.  

the answer is a, salt plain