Answer:
B. that the stars gradually disappear
Step-by-step explanation:
According to the sentence from the section Two Key Parts Make Up Nuclear Fusion, "this means the end of the line for small
stars, who quietly fade into darkness after they have used up their hydrogen", the phrases "end of the line" and "fade into darkness" makes the reader understand that the stars gradually disappear.
This is because, the phrases show that the stars will gradually disappear as they have run out of hydrogen.