Which is the best way to combine the sentences using a semicolon? "The world’s nations must cooperate. One nation cannot solve the world’s problems by itself. " Explain with your reasoning. 
A. The world’s nations must cooperate; for one nation cannot solve the world’s problems by itself.
B. The world’s nations must cooperate; one nation cannot solve the world’s problems by itself.
C. The world’s nations must cooperate; One nation cannot solve the world’s problems by itself.