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.