Critical reading – what do Mariner 9 images imply about Mars? The passage: Mariner 9 saw valley networks like Earth’s stream beds, implying Mars once had a thicker, heat-trapping atmosphere; if true, something stripped it away billions of years ago. What statement is best supported?

Introduction / Context:
The paragraph links surface features to past climate, then reasons that if a thick atmosphere once existed, it must have been removed long ago. The best-supported claim will reflect the implied present state.

Given Data / Assumptions:

• Valley networks resemble stream beds on Earth.
• This suggests a past atmosphere thick enough to retain heat.
• Something stripped Mars’s atmosphere billions of years ago.

Concept / Approach:
If the atmosphere was thick and then stripped, the current atmosphere must be greatly reduced (little or none). The text does not quantify Earth–Mars comparisons, first photos, orbital positions, or topography.

Step-by-Step Solution:
From “once thick” + “stripped away” ⇒ present state is thin or nearly absent.Map to option A precisely.Eliminate options that add comparisons or facts not stated (thicker than Earth, first pictures, orbital distance, mountains).

Verification / Alternative check:
Restate: If X existed and was removed, X is now minimal—this is the only conclusion tied directly to the text.

Why Other Options Are Wrong:

• B: The passage claims “thick enough,” not “thicker than Earth.”
• C/D/E: Unmentioned facts; not inferable.

Common Pitfalls:
Over-reading comparisons with Earth or inventing “first” claims; stick to the explicit implication about the current atmosphere.

Final Answer:
Mars now has little or no atmosphere.