Which of the following statements is​ correct? A. The relationship between two variables is linear whether it is represented by a straight line or by a curved line. B. The relationship between two variables is nonlinear whether it is represented by a straight line or by a curved line. C. The relationship between two variables is linear when it is represented by a straight line and nonlinear when it is represented by a curved line. D. The relationship between two variables is linear when it is represented by a curved line and nonlinear when it is represented by a straight line.