Select the correct options about photopigments from the following statements. (a)   Rods are specialised for photopic vision and cones are specialised for scotopic vision.(b)   The rods contain rhodopsin or visual purple.(c)   In the human eye, there are three types of cones which respond optimally to red, green and blue lights.(d)   A sensation of white light is produced when three types of cones are stimulated equally.