Question

the nose knows
many sharks swim long distances around
the planet. how they do this without getting
lost is a mystery. a group of scientists in
california thought the sense of smell could
be an answer. to find out, they stuffed the
noses of one group of sharks with cotton
balls. a second groups noses remained
unplugged. both groups were taken to a
deep part of the ocean. the sharks that
didnt have cotton balls in their noses swam
straight toward shore. those with cotton balls seemed to wander. therefore, the
scientists believe that sharks may indeed use their noses to get around in the ocean.
what is the main, or central, idea of the passage?
to study sharks sense of smell, scientists stuffed cotton balls in the animals
noses.
scientists discovered that sharks find their way around by using their noses.
scientists know that sharks can swim long distances without getting lost.

Explanation:

The passage describes an experiment where scientists tested if sharks use their sense of smell (noses) to navigate. The key result is that sharks with plugged noses wandered, while those with unplugged noses swam straight. The main idea is the discovery about sharks' navigation using noses, not just the experiment setup (first option) or a known fact (third option which contradicts the "mystery" in the passage).

Answer:

Scientists discovered that sharks find their way around by using their noses.