The correct answer is the last one:
A and C are different elements, while D is an isotope of C.
In fact, A and C are different elements, because they have a different number of protons in the nucleus (A has 3 protons, while C has 4 protons). Instead, D and C are the same element (they both have 4 protons in the nucleus), but they are different isotopes since they have a different number of neutrons (D has 4 neutrons while C has 3 neutrons)
Gravity
The moon doesn't smash into the earth because the gravity from the earth keeps the moon in orbit around it.
If you are talking about volume, then an easy way to measured the volume of a liquid would be with a graduated cylinder. A graduated cylinder is marked with volume units such as milliliter (mL) or the liter (L). One liter equals 1 thousand milliliters.
To measure the volume of a solid you use the formula V = (Length)(Width)(Height)
To measure the volume of a gas you use a graduated cylinder held upside down. At first the cylinder is filled with water. When air is blown into the cylinder, bubbles rise and push some water down. The volume of the water pushed down is equal to the volume of the gas that was blown in.
