Question

the mysterious glow of the firefly
fireflies (also called lightning bugs) are found in many damp regions of asia and the americas. there are about 2,000 different species of firefly. but how do these insects, which are actually a type of beetle, produce light, and why do they do it? under the fireflys abdomen there is a special organ for producing light. when the insect takes in oxygen, it is combined in special cells with a substance called luciferin, and this makes that part of their bodies glow. this light gives off almost no heat.
the fireflies glow is necessary for their survival. for one thing, the blinking light is used to lure their prey. it is also used as a warning sign to scare off predators. perhaps the most essential purpose of the fireflys glow is to attract a mate. if you ever watch fireflies, you will see that their light flashes in various patterns, rather than steadily. each species has a unique pattern of flashing. because of this unique pattern, fireflies are able to find mates within their own species. scientists have not yet discovered how fireflies can turn the glow on and off
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what causes the firefly to be able to produce light?
a. a unique pattern of flashing
b. a substance called luciferin
c. about 2,000 different species

Explanation:

The text states that when the firefly takes in oxygen, it combines with a substance called luciferin in special cells to make the body part glow. Option A (unique pattern of flashing) is about how they use the light (e.g., for mating) not what causes the light. Option C (2000 species) is just a fact about fireflies, not related to the cause of light production.

Answer:

B. a substance called luciferin