Why does in the building of an atom, the filling of 4s orbital takes place before filling in 3d orbital ?

Assertion : Ionisation of transition metals involves loss of ns electron before (n - 1)d electrons. Reason : Filling of ns-orbitals takes placed before (n - 1)d orbitals.

After filling the 4d-orbitals, an electron will enter in:

While filling up of electrons in the atomic orbitals, the 4s orbital is filled before the 3d orbital but reverse happens during the ionisation of the atom. Explain why?

4s orbitals has less energy than 3d orbital

Assertion : In transition elements radii of 5d series are virtually the same as those of the corresponding members of 4d series. Reason : The filling of 4f orbitals before 5d orbitals results in regular decrease in atomic radii.