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Verbal·Information and Ideas·Inferences

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When searching for atmospheric biosignatures on exoplanets, astrobiologists prioritize chemical disequilibria—the simultaneous presence of gases that would rapidly react and destroy each other unless continuously replenished by biological activity. For example, detecting methane ${\mathrm{CH}}_4$ in an oxygen-rich ${\mathrm{O}}_2$ atmosphere is considered strong evidence of life because photochemical reactions driven by stellar ultraviolet radiation break down methane into water and carbon dioxide within decades. However, planetary modeler Dr. Kevin Zahnle cautions that finding atmospheric methane alone, in the absence of abundant oxygen or ozone, cannot serve as a reliable biosignature because       

Which choice most logically completes the text?

A

abiotic processes, such as serpentinization and volcanic outgassing, can sustain high methane levels on lifeless rocky worlds.

B

oxygen and methane cannot coexist in any planetary atmosphere regardless of biological activity.

C

microbial methanogens on Earth do not release methane into the atmosphere in measurable quantities.

D

ultraviolet radiation from stars accelerates biological photosynthesis while suppressing abiotic geologic emissions.