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. 

AThe world’s nations must cooperate; for one nation cannot solve the world’s problems by itself.

BThe world’s nations must cooperate; one nation cannot solve the world’s problems by itself.

CThe world’s nations must cooperate; One nation cannot solve the world’s problems by itself.