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

[Q]: Passage: The alkaline earth metals include beryllium, magnesium, calcium, strontium, and barium. Strontium and barium react with air and must be stored in oil. Answer: alkaline earth metals
[A]: Beryllium, magnesium, calcium, strontium, and barium are classified as what type of metals?

[Q]: Passage: Sliding friction is friction that acts on objects when they are sliding over a surface. Sliding friction is weaker than static friction. That's why it's easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Sliding friction can be useful. For example, you use sliding friction when you write with a pencil. The pencil "lead" slides easily over the paper, but there's just enough friction between the pencil and paper to leave a mark. Answer: sliding friction
[A]: What type of friction is friction that acts on objects when they are sliding over a surface?

[Q]: Passage: Heterotrophs cannot make their own food, so they must eat or absorb it. For this reason, heterotrophs are also known as consumers . Consumers include all animals and fungi and many protists and bacteria. They consume either autotrophs or other heterotrophs. Heterotrophs show great diversity and may appear far more fascinating than producers. But heterotrophs are limited by their utter dependence on those autotrophs which originally made the food. If plants, algae, and autotrophic bacteria vanished from Earth, animals, fungi, and other heterotrophs would soon disappear as well. All life requires a constant input of energy. Only autotrophs can transform that ultimate, solar source into the chemical energy in food which powers life, as shown in Figure below . Answer: consumers
[A]:
Because heterotrophs cannot make their own food, what are they called?