Economics · Production Theory - Ridge Lines

Which of the following statements regarding Ridge Lines in production theory are correct? A. Ridge lines denote the boundaries of the economically rational region of production. B. The upper ridge line connects points where the Marginal Rate of Technical Substitution (MRTS) is infinite and the marginal product of labour is zero. C. Operating outside the region defined by the ridge lines implies that a firm is employing inputs with zero or negative marginal products. D. Beyond the upper ridge line, isoquants bend backward and acquire a positive slope.