Main sequence starts maybe
It means “the study of.” Think of it:
Those are subjects you study. It makes more sense than the other choices.
Ionic compounds are between metals and non metals, otherwise if it’s a nonmetal with a nonmetal it’s molecular
It has more lines in it compared to hydrogen emission spectrum. It is mainly because the helium atom has more electrons than a hydrogen atom. Therefore, more electrons get excited when we pass a white light beam through a helium sample, and it causes the emission of more spectral lines
<u><em>THEY ARE NOT AFFECTED BY EARTH`S ATMOSPHERE</em></u>
