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

Passage: A variable is a factor that can change over the course of an experiment. Independent variables are factors whose values are controlled by the experimenter to determine its relationship to an observed phenomenon (the dependent variable). Dependent variables change in response to the independent variable. Answer: independent variables
The values of which factors are controlled by the experimenter over the course of an experiment?

Passage: It's much easier to precisely make mirrors than to precisely make glass lenses. For that reason, reflectors can be made larger than refractors. Larger telescopes can collect more light. This means that they can study dimmer or more distant objects. The largest optical telescopes in the world today are reflectors. Telescopes can also be made to use both lenses and mirrors. Answer: destroyed
What happens to old oceanic crust at convergent boundaries?

Passage: Other dry climates get a little more precipitation. They are called steppes . These regions have short grasses and low bushes (see Figure below ). Steppes occur at higher latitudes than deserts. They are dry because they are in continental interiors or rain shadows. Answer: steppes
What is the term for dry climate regions at higher altitudes that have short grasses and low bushes?