Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: The frost line of the solar system divides the terrestrial plants from the gas-giant planets. What elements make up most of the core of the planets orbiting outside of the frost line?
Hydrogen and Helium.