Question

passage
ancient sea salt drilled from a geologic basin in russia is providing dramatic new clues as to how earths early atmosphere became oxygen - rich—allowing life as we know it to evolve. buried deep beneath the surface for billions of years, the salt reveals surprising clues about the chemistry of the ocean and atmosphere from long ago.
the salt, excavated by an international team led by russian scientists, is about a billion years older than other, similar geologic samples. its age puts it smack in the middle of earths great oxygenation event, the ancient period in which oxygen began to dominate atmospheric chemistry. “this is a truly unique, one - of - a - kind deposit,” says clara blättler, a geochemist at princeton university. the salty new samples are made up of minerals left behind when water evaporates. “because these evaporite minerals are our most direct way of sampling ancient sea waters, this deposit gives us a snapshot of seawater in the interval time when we dont really have any other direct constraints.”
from nola taylor redd, “primeval salt shakes up ideas on how the atmosphere got its oxygen.” ©2018 by scientific american, a division of nature america, inc.
question
one central idea of the passage is that
the air is a by - product of the process that deposits salt in earth’s oceans
salt deposits provide evidence of the way the ancient oceans and air interacted
international cooperation is at the heart of many of science’s best discoveries
atmospheric chemists are most interested in the earth’s great oxygenation event

Explanation:

The passage focuses on ancient sea salt from Russia. It mentions that the salt gives clues about the chemistry of the ocean and atmosphere long ago. The salt is made of evaporite minerals which are a direct way to sample ancient seawaters. This indicates that the salt deposits can show how the ancient oceans and air (atmosphere) interacted.

• The first option about air being a by - product of salt - depositing process is not supported. The passage is about using salt to study the atmosphere, not about the air being a by - product of salt deposition.
• The third option about international cooperation is a minor detail (only mentioned that the salt was excavated by an international team) and not the central idea.
• The fourth option about atmospheric chemists' interests is not the main focus. The main focus is on the salt deposits and what they reveal, not just the chemists' interest in the Great Oxygenation Event.

Answer:

salt deposits provide evidence of the way the ancient oceans and air interacted