instruction:
Given a scientific passage and an answer, generate a question for the given answer.
question:
Passage: To this point, we have examined the relationships between any two of the variables of , , and , while the third variable is held constant. However, situations arise where all three variables change. The combined gas law expresses the relationship between the pressure, volume, and absolute temperature of a fixed amount of gas. For a combined gas law problem, only the amount of gas is held constant. Answer: pressure
answer:
The combined gas law involves three properties of a gas - volume, absolute temperature, and what?

question:
Passage: The cell membrane is made up of a phospholipid bilayer, two layers of phospholipid molecules. Notice the polar head group of the phospholipid is attached to the phosphate, and the tails are two fatty acid chains. The head group and tails are attached by a glycerol backbone. Answer: cell membrane
answer:
What basic cell structure is made up of a phospholipid bilayer and two layers of phospholipid molecules?

question:
Passage: Glaciers and ice sheets are melting. This can be seen clearly in photographs. The glaciers advanced during the Pleistocene ice age. They have been melting back since then, except for during the Little Ice Age. However, the rate of melting has increased tremendously in recent decades. Answer: pleistocene ice age
answer:
Glaciers have been melting since what period?