Oxygen has Atomic number 8 so all isotopes have 8 protons and 8 electrons.
So the number of neutrons in Oxygen-18 = 18 - 8 = 10.
Option B is the correct one.
I would say Book B.
Gravitational Potential Energy is mass * gravity * height.
Gravity is same for the same, it's constant.
Book C is the lowest, so it has less height.
And Book B is clearly the more massive of Book A and Book B, which are the same height.
Book B
The answer is False, they are not always correct
It will take "Exact 75 years", 'cause light takes 1 year for 1 light years.
Hope this helps!