Answer: B) Dhana Nanda Explanation:

Answer: A) Total internal reflection Explanation: A periscope is an optical instrument that uses a system of prisms, lenses, or mirrors to reflect images through a tube.       Light from a distant object strikes the top mirror and is then reflected at an angle of 90 degrees down the periscope tube. At the bottom of the periscope, the light strikes another mirror and is then reflected into the viewer’s eye.   Hence, A periscope works by the principle of Total internal reflection.

Answer: C) 4 Explanation: Let the number of boys in the group originally be x.Total weight of the boys = 30xAfter the boy weighing 35 kg joins the group, total weight of boys = 30x + 35So 30x + 35 + 31(x + 1) = > x = 4.

Answer: C) 3 Explanation: The given figure can be labelled as shown : The spaces P, Q and R have to be shaded by three different colours definitely (since each of these three spaces lies adjacent to the other two).Now, in order that no two adjacent spaces be shaded by the same colour, the spaces T, U and S must be shaded with the colours of the spaces P, Q and R respectively.Also the spaces X, V and W must be shaded with the colours of the spaces S, T and U respectively i.e. with the colours of the spaces R, P and Q respectively. Thus, minimum three colour pencils are required.