Answer: D) 21 Explanation: Total number of exams written by 100 students = 48 + 45 + 38 = 131Now let us say x members are writing only 1 exam, y members are writing only 2 exams, z members are writing only 3 exams.Therefore, x + 2y + 3z = 131 also x + y + z = 100.Given that z = 5. So x + 2y = 116 and x + y = 95.Solving we get y = 21.So 21 members are writing exactly 2 exams.

Answer: B) Stratosphere Explanation: The ozone layer is a region of Earth's Stratosphere that absorbs most of the Sun's ultraviolet (UV) radiation. It contains high concentrations of ozone (O3) in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere.   The Atmosphere is divided into 4 layers according to major changes in temperature. They are : 1. Troposphere, 2. Stratosphere, 3. Mesosphere  and 4. Thermosphere. The farther away from earth, the thinner the atmosphere gets.