How Do Planetary Scientists Explain the Small Amounts of Hydrogen

How do planetary scientists explain the small amounts of hydrogen and helium in Mercury's atmosphere?

A) Mercury's large iron core has allowed it to retain these light gases from its primordial atmosphere.
B) They have escaped from the Sun and drifted to Mercury. Mercury does not retain them, but they are continually replenished from the Sun.
C) They are outgassed from Mercury's active volcanoes.
D) The hydrogen at least is formed when the Sun's intense ultraviolet radiation breaks apart the hydrocarbons on Mercury's surface.