Why Was Moseley So Sure from His X-Ray Work That

Why was Moseley so sure from his X-ray work that there were no other undiscovered elements besides the three that he predicted in the range of atomic numbers 13-79?

A) The rows on the periodic table were filled.
B) The columns of the periodic table were full.
C) All of the possible atomic weights were assigned.
D) All of the integer atomic numbers were assigned.
E) Mendeleev didn't predict any more.