Underwater, divers must breathe a combination

The correct answer that would best complete the given statement above would be OXYGEN and HELIUM. Or in other terms, it is known as HELIOX. Heliox is the mixture of oxygen and helium and it has a lesser density than air. Using heliox for breathing decreases the resistance to flow within the airways and also decreases the work of breathing. Hope this answer helps.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.