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

[EX Q]: Passage: Carbon has four valence electrons, so it can achieve a full outer energy level by forming four covalent bonds. When it bonds only with hydrogen, it forms compounds called hydrocarbons. Answer: four
[EX A]: How many valence electrons does carbon have?

[EX Q]: Passage: If water is so simple, why is it special? Water is one of the few substances that exists on Earth in all three states of matter. Water occurs as a gas, a liquid and a solid. You drink liquid water and use it to shower. You breathe gaseous water vapor in the air. You may go ice skating on a pond covered with solid water ice in the winter. Answer: three matter states
[EX A]: In how many states does water exist on earth?

[EX Q]: Passage: You could argue that oxygen is one of the most important, if not THE most important molecule necessary for life. However, oxygen is essentially just a waste product of the light reactions of photosynthesis. It is a "leftover" from a necessary part of the process. All the oxygen that is necessary to maintain most forms of life just happens to be released from the plant during this process. Answer: photosynthesis
[EX A]:
Oxygen is essentially just a waste product of the light reactions of what?