Given a scientific passage and an answer, generate a question for the given answer.
--------
Question: Passage: Figure 12.7 Statues made from carbonate compounds such as limestone and marble typically weather slowly over time due to the actions of water, and thermal expansion and contraction. However, pollutants like sulfur dioxide can accelerate weathering. As the concentration of air pollutants increases, deterioration of limestone occurs more rapidly. (credit: James P Fisher III). Answer: air pollutants

Answer: Deterioration of limestone occurs more rapidly as the concentration of what increases?

Question: 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

Answer: Cnidarians have a simple digestive system with a single opening surrounded by what type of structures, which are used to capture prey?

Question: Passage: Coal is a black or brownish-black rock that burns easily ( Figure below ). Most coal is sedimentary rock. The hardest type of coal, anthracite, is a metamorphic rock. That is because it is exposed to higher temperature and pressure as it forms. Coal is mostly carbon, but some other elements can be found in coal, including sulfur. Answer: carbon

Answer:
What element makes up the majority of coal?