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

[Q]: Passage: The body of a cnidarian consists of two cell layers, ectoderm and endoderm. The cells surround a digestive cavity called the coelenteron (see Figure below ). Cnidarians have a simple digestive system. The single opening is surrounded by tentacles, which are used to capture prey. The tentacles are covered with nematocyst cells. Digestion takes place in the coelenteron. Nutrients are absorbed and gases exchanged through the cells lining this cavity. Fluid in the coelenteron creates a hydrostatic skeleton. Answer: tentacles
[A]: Cnidarians have a simple digestive system with a single opening surrounded by what type of structures, which are used to capture prey?

[Q]: Passage: A single-replacement reaction is one in which an element replaces another element in a compound. An element is in either the solid, liquid, or gas state and is not an ion. The example below shows the reaction of solid magnesium metal with aqueous silver nitrate to form aqueous magnesium nitrate and silver metal. Answer: single-replacement reaction
[A]: In this type of reaction, an element replaces another element in a compound, and the element is in any state of matter but is not an ion?

[Q]: Passage: Meanders form because water erodes the outside of curves and deposits eroded material on the inside. Over time, the curves shift position. Answer: meanders
[A]:
What form because water erodes the outside of curves and deposits eroded material on the inside?