What is drag? In what situations might it act on the force of gravity? (Please don´t use to many big words- it gets confusing ;-;)

Henry moseley used x-ray experiments to determine the atomic number of elements. how did his discovery contribute to the development of the periodic table? a.it confirmed that elements should be arranged in strict order of increasing atomic mass. b.it led to elements with similar atomic numbers being grouped together. c.it allowed the elements to be placed in strict order of increasing atomic number. d.it showed that the way mendeleev grouped elements together was completely wrong.