The Energy Required to Remove an Electron from a Helium

The energy required to remove an electron from a helium ion, He⁺, (8.72 x 10^-18 J) is more than twice the energy required to remove an electron from a helium atom (3.94 x 10^-18 J) . The reason(s) are

A) 1s electrons screen each other.
B) the helium ion has a larger Z.
C) the helium atom has less electron-electron repulsions.
D) the measurement is compromised by the Heisenberg uncertainty principle.
E) helium ions have smaller nuclei.