For decades, the prevailing view in cosmochemistry held that Earth's water arrived late, delivered by icy comets after the planet had largely formed. Recent isotopic analyses of ancient zircon crystals, however, suggest that water was present much earlier — perhaps incorporated into Earth's building blocks from the start. This does not rule out cometary contributions, but it shifts the burden of explanation: the question is no longer where water came from, but how much arrived when.
Which choice best states the main idea of the text?
- Acheck_circle
New evidence has revised, though not entirely overturned, the standard account of Earth's water origins.
- B
Cosmochemists have abandoned the late-delivery hypothesis.
- C
Zircon crystals are the most reliable record of Earth's early history.
- D
Comets did not deliver any water to Earth.
Explanation
The passage acknowledges the older view, presents revising evidence, and explicitly notes comets aren't ruled out — B captures this nuance. A and D overstate; C is unsupported.