The correct answer is “It was hard, being captain of the constellation"
Step-by-step explanation:
In this paragraph, the narrator explains the thoughts of Astrid as she is leaving the moon-ball court. In this, the narrator explains her main conflict or problem. This is introduced in "But that was nothing compared to the heavyweight in her shoulders" because this detail shows feeling pulled by the gravity is not as important or problematic as the main problem Astrid is dealing with.
Moreover, this big problem is explained in the sentence "It was hard, being captain of the constellation" because this explains the cause why she feels a heavy weight on her shoulders or why she is going through a difficult situation. According to this, the sentence that explains that Astrid had a big problem and clarifies the idea of "heavy weight in her shoulders" is "It was hard, being captain of the constellation."