An electron, practically at rest, is initially accelerated through a potential difference of 100 volts . It then has a de Broglie wavelength = A. It then gets retarded through 19 volts and then has wavelength A. A further retardation through 32 volts changes the wavelength to . What is

 a) 10/63 b) 20/63 c) 40/63 d) 1/63 solution:b) 20/63