Electrons are accelerated from rest through an electric potential difference. (a) If this potential difference increases to

Question:

Electrons are accelerated from rest through an electric potential difference.
(a) If this potential difference increases to four times the original value, the new de Broglie wavelength will be (1) four times, (2) twice, (3) one-fourth, (4) one-half that of the original. Why?
(b) If the original potential is 250 kV and the new potential is 600 kV, what is the ratio of the new de Broglie wavelength to the original?