4)   Radio waves
     (a)  1, 3, 2, 4
     (b)  3, 2, 4, 1
     (c)  4, 1, 3, 2
     (d)  4, 3,1, 2
Ans : (c)
22. Nuclear fission is caused by the impact of: [RRB 1991]
     (a) proton
     (b) electron
     (c) neutron
     (d) None of these
Ans : (c)
23. Which of the following is used as a coolant in nuclear reactors?
     (a) Heavy water
     (b) Cadmium
     (c) Liquid "sodium
     (d) Graphite
Ans : (c)
24. Which of the following is good nuclear fuel?
     (a) Uranium-238
     (b) Neptunium-239
     (c) Thorium-236
     (d) Plutonium-239
Ans : (d)
25. What is the spectrum?
     (a) 1300-3000 Å
     (b) 3900-7600 Å
     (c) 7800-8000 Å
     (d) 8500-9800 Å
Ans : (b)
26. In an atomic explosion, enormous energy is released which is due to the: [Railways 1992]
     (a) conversion of neutrons into protons
     (b) conversion of chemical energy into heat energy
     (c) conversion of chemical energy into nuclear energy
     (d) conversion of mass in to energy
Ans : (d)
27. Which of the following statements are correctly associated with ultrasonic waves? [CDS 1990]
     1) These are the acoustic waves of frequencies more than 20,000Hertz.
     2) When possible to use, these waves are preferred over X-rays for getting an image of internal organs because they are not
          harmful.
     3) Their velocity is more than the velocity of sound.
     (a) 1only
     (b) 1and 2
     (c) 2 and 3
     (d) 1and 3
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