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

Example Input: Passage: Wavelength is one way of measuring the size of waves. It is the distance between two corresponding points on adjacent waves, and it is usually measured in meters. How it is measured is a little different for transverse and longitudinal waves. Answer: waves
Example Output: What do wavelengths measure the size of?

Example Input: Passage: Figure 32.21 Supernovas are the source of elements heavier than iron. Energy released powers nucleosynthesis. Spectroscopic analysis of the ring of material ejected by Supernova 1987A observable in the southern hemisphere, shows evidence of heavy elements. The study of this supernova also provided indications that neutrinos might have mass. (credit: NASA, ESA, and P. Challis). Answer: supernovas
Example Output: What celestial events are the source of elements heavier than iron?

Example Input: Passage: All life relies on a relatively narrow range of pH, or acidity. That's because protein structure and function are very sensitive to pH. Air pollution can cause precipitation to become acidic. Nitrogen and sulfur oxides, mainly from motor vehicle exhaust and coal burning, create acids when they combine with water in the air. The acids lower the pH of precipitation, forming acid rain . If acid rain falls on the ground, it may damage soil and soil organisms. If it falls on plants, it may kill them (see Figure below ). If it falls into lakes, it lowers the pH of the water and kills aquatic organisms. Answer: acid rain
Example Output:
When the ph of precipitation is lowered because of air pollution, what results?