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

[EX Q]: Passage: Momentum is a vector quantity that has the same direction as the velocity of the object. Answer: object ' s velocity
[EX A]: Momentum is a vector quantity that has the same direction as what?

[EX Q]: Passage: A snowflake is made of ice, or water in the solid state. A solid is one of four well-known states of matter. The other three states are liquid, gas, and plasma. Compared with these other states of matter, solids have particles that are much more tightly packed together. The particles are held rigidly in place by all the other particles around them so they can't slip past one another or move apart. This gives solids a fixed shape and a fixed volume. Answer: plasma
[EX A]: What state of matter completes the list: solid, liquid, gas?

[EX Q]: Passage: The major cause of outdoor air pollution is the burning of fossil fuels. Fossil fuels are burned in power plants, factories, motor vehicles, and home heating systems. Ranching and using chemicals such as fertilizers also cause outdoor air pollution. Erosion of soil in farm fields, mining activities, and construction sites adds dust particles to the air as well. Some specific outdoor air pollutants are described in Table below . Answer: burning fossil fuels
[EX A]:
What is the major cause of outdoor air pollution?