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

Q: Passage: As pictured below ( Figure below ), some grazing animals, especially sheep and goats, eat grass right down to the roots. They may even pull the grass entirely out of the ground. Grazing animals can kill the grass. They may thin it out so much that it offers little protection to the soil. If animals are kept in the same place too long, the soil may become completely bare. The bare soil is easily eroded by wind and water. Answer: erosion

A: By pulling out grass by its roots and over-grazing, livestock contribute to what negative process?
****
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

A: How many valence electrons does carbon have?
****
Q: Passage: As the fast-moving cold air mass keeps advancing, so does the cold front. Cold fronts often bring sudden changes in the weather. There may be a thin line of storms right at the front that moves as it moves. In the spring and summer, these storms may be thunderstorms and tornadoes. In the late fall and winter, snow storms may occur. After a cold front passes, the cold air mass behind it brings cooler temperatures. The air is likely to be less humid as well. Can you explain why?. Answer: snow storm

A:
Cold fronts in winter may bring what type of storm?
****