(a) Neutron and Proton
   (b) Electron and Proton
   (c) Neutron and Electron
   (d) Proton, Neutron and Electron
Ans: (a)
339. For an electron orbiting in a hydrogen atom, the necessary centripetal
   force keeping it in orbit is provided by
   (a) the electrical attraction between the negatively charged electron and the
       positively charged porton
   (b) the high speed of the electron
   (c) the low speed of the electron
   (d) gravity
Ans: (a)
340. The filament lamp is an example for
   (a) fluorescence
   (b) incandescence
   (c) both (a) and (b)
   (d) None of these
Ans: (b)
341. Out of the following, name the fluorescent substance.
   (a) quinine sulphate solution
   (b) paraffin oil
   (c) fluorescein solutions
   (d) All the above
Ans: (d)
342. Atoms which have the same atomic number but different mass numbers
   are called
   (a) isobars
   (b) isomers
   (c) isotones
   (d) isotopes
Ans: (d)