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

[EX Q]: Passage: Population density is the average number of individuals in a population per unit of area or volume. For example, a population of 100 insects that live in an area of 100 square meters has a density of 1 insect per square meter. If the same population lives in an area of only 1 square meter, what is its density? Which population is more crowded? How might crowding affect the health of a population?. Answer: population density
[EX A]: What is the term for the average number of individuals in a population per unit of area or volume?

[EX Q]: Passage: Some chemical reactions can occur in only one direction. These reactions are called irreversible reactions. The reactants can change to the products, but the products cannot change back to the reactants. These reactions are like making a cake. The ingredients of a cakesuch as eggs and flourare the reactants. They are mixed together and baked to form the cake, which is the product (see Figure below ). The cake can't be "unbaked" and "unmixed" to change it back to the raw eggs, flour, and other ingredients. So making a cake is irreversible. Answer: irreversible
[EX A]: What are reactions that can occur only in one direction called?

[EX Q]: Passage: Some regions of the electron cloud are denser than others. The denser regions are areas where electrons are most likely to be. These regions are called orbitals . Each orbital has a maximum of just two electrons. Different energy levels in the cloud have different numbers of orbitals. Therefore, different energy levels have different maximum numbers of electrons. Table below lists the number of orbitals and electrons for the first four energy levels. Energy levels farther from the nucleus have more orbitals. Therefore, these levels can hold more electrons. Answer: orbitals
[EX A]:
The denser regions of the electron cloud are called what?