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

Input: Consider Input: Passage: Natural gas is delivered to homes, where it is used for cooking and heating. Natural gas is also a major energy source for powering turbines to make electricity. Natural gas releases most of its energy as heat when it burns. The power plant is able to use this heat, either in the form of hot gases or steam, to spin turbines. The spinning turbines turn generators, and the generators create electricity. Answer: natural gas

Output: What type of gas is delivered to homes to use for cooking and heating?

Input: Consider Input: Passage: All the atoms of a given element have the same number of protons in their nucleus, but they may have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes. Many elements have one or more isotopes that are radioactive. These isotopes are called radioisotopes . Their nuclei are unstable, so they break down, or decay, and emit radiation. Answer: protons

Output: All the atoms of a given element have the same number of what in their nucleus, though they may have different numbers of neutrons?

Input: Consider Input: Passage: ; The efficiency equals the output power divided by the input power. Answer: efficiency
Output: What equals the output power divided by the input power?