The double-slit electron interference pattern appears when a large number of electrons are accumulated at the screen. According to the quantum theory, the interference pattern appears
(1) only if a beam of electrons are in transit so that one part of the beam goes through one slit and another part through the other
(2) even if one electron at a time is in transit, but an electron goes through one of the two slits only
(3) even if one electron at a time is in transit, but an electron goes through both the slits at once
(4) the consideration of an electron at a time in transit is not physically possible
