How Should We Expect the Milky Way's Interstellar Medium to Be

How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?

A) The total amount of gas will be much less than it is today.
B) The total amount of gas will be about the same, but the percentage of elements heavier than hydrogen and helium will have risen from the current 2% to more than 50%.
C) The total amount of gas will be much greater, since many stars will undergo supernovae between now and then.
D) Thanks to the recycling of the star-gas-star cycle, the interstellar medium should look about the same in 50 billion years as it does today.