According to readings from the Mauna Loa Observatory, CO₂ concentrations in May 2013 exceeded ________ for the first time during the last 800,000 years.
A) 100-ppm
B) 200-ppm
C) 300-ppm
D) 400-ppm

Question

Quizplus · Accepted Answer

The question states that CO₂ concentrations exceeded a level for the first time in 800,000 years, which rules out choices A, B, and C as these levels have been surpassed in the recent past. The correct answer is therefore D, 400-ppm, which was last seen during the Pliocene Epoch, 3 million years ago.

According to Readings from the Mauna Loa Observatory, CO₂ Concentrations