Which of the following statements are correct? (a) For a lens, the first principal focus is the position of the object whose image is at infinity. (b) For a lens, the first principal focus is the position of the object whose image is formed at twice the focal length. (c) For a lens, the second principal focus is the position of the real image whose object is at infinity. (d) For a lens, the second principal focus is the position of the object whose image is formed at twice the focal length.