A, B only A, B, C only B, C only B, C, D only

B, C, D only

In (A) , Focal length after cutting it from middle becomes double. i.e after cutting it becomes 40cm. In case (B) Focal length remains same. i.e f=20cm. A lens will change its nature if its put in a liquid of higher refractive index then its nature will change i.e convex lens become concave. (B) , (C) , (D) are correct.