Question

read the passage. then answer the question.
a galaxy contains billions of stars and their solar systems.
each solar system contains a different number of planets.
when scientists consider which planets might be able to
support life, they ask two questions. does the planet have
water, and does it have an energy source, like a sun or
volcanoes?
both water and an energy source are needed for the chemical
reactions to take place that make life possible. without those
ingredients, the chemicals dont have what they need to
change.
what is a characteristic of an energy source as it is used in
the passage?
a gives off heat
b exists on every planet
c needs water
d unimportant for life

Explanation:

The passage mentions that an energy source like a sun or volcanoes is needed. A sun gives off heat, and volcanoes also release heat. Option B is incorrect as not every planet has such an energy source. Option C is wrong as the energy source (like sun/volcano) doesn't need water. Option D is incorrect because the passage states both water and energy source are needed for life - making chemical reactions.

Answer:

A. gives off heat