What is the atomic size going across a period (left or right)? what causes this trend?

The atomic size decreases; this is because as you go across the period, more electrons are found in the outer shell. As more electrons are added, the more attraction there is between the protons, in the nucleus, and the electrons, making the atom smaller.
Hope this helps!

Ibelieve its a because an empirical formula usually has a ch in it your options are a or b