20. In which of the following media is speed of Sound the least? A. Air B. Brass C. Water D. Wood 21. Which of the following characteristics of waves can a ripple tank be used to demonstrate? I. Reflection II. Refraction III. Diffraction A. I, II and III B. II and III only C. I and III only D. I and II only 22. Which of the following graphs gives the correct relationship between energy and mass when mass is converted to energy. 23. An 800 kg car moving at 80 km hr-1 collides with a 1200 kg car moving at 40 km hr-1 in the same direction. If the cars stick together, calculate their common velocity. A. 60 km hr-1 B. 8 km hr-1 C. 40 km hr -1 D. 56 km hr-1 24. The mass of a load is doubled while the force acting on it is halved. The resulting acceleration of the load is A. quadrupled. B. quartered. C. halved. D. doubled. 25. The maximum and minimum thermometer reads the A. maximum temperature during the day and the minimum temperature at night at all times. B. maximum temperature at night and minimum temperature during the day from the last reset. C. maximum temperature at night and minimum temperature during the day at all times. D. maximum temperature during the day and minimum temperature at night from the last reset. 26. An a.c. generator can be converted to a d.c. generator by replacing the A. commutator with an armature. B. armature with a commutator. C. commutator with slip rings. D. slip rings with a commutator.