Given a scientific passage and an answer, generate a question for the given answer.

[Q]: Passage: Figure 23.30 Virtually all aquatic organisms depend directly or indirectly on protists for food. (credit "mollusks": modification of work by Craig Stihler, USFWS; credit "crab": modification of work by David Berkowitz; credit "dolphin": modification of work by Mike Baird; credit "fish": modification of work by Tim Sheerman-Chase; credit "penguin": modification of work by Aaron Logan). Answer: protists
[A]: Virtually all aquatic organisms depend directly or indirectly on what for food?

[Q]: Passage: Carbon moves through ecosystems in two cycles that overlap. In the biotic cycle, it moves between living things and the air. In the abiotic cycle, it moves between the air, ground, and oceans. By burning fossil fuels, humans have increased the amount of carbon dioxide in the air. Answer: biotic cycle
[A]: In what part of the carbon cycle does carbon move between living things and the air?

[Q]: Passage: Star clusters are small groups of stars. A star cluster is smaller than a galaxy. There are two main types, open clusters and globular clusters. Both types are held together by gravity. Answer: gravity
[A]:
What force holds together both types of star clusters?