[" 20.A man at the bottom of a pool wants to signal to a "],[" person lying at the edge of the pool.The man should "],[" beam his water proof light "],[" (a) vertically upwards "],[" (b) at an angle to the vertical which is less than the "],[" critical angle."],[" (c) at an angle to vertical which is equal to the criti- "],[" cal angle "]