Which of the following is evidence for how the composition of the atmosphere has changed on Earth over time?
a. Bands of iron found in rock layers show that the Earth's atmosphere became oxygenated by photosynthetic cyanobacteria millions of years ago.
b. Frequent volcanic eruptions have caused the Earth's atmosphere to fill with sulfur aerosols dropping the temperature globally by 5 degrees.
c. Multiple ice ages in the 18th century have caused a shift in atmospheric circulation on present day Earth.
d. Over the past 50 years, greenhouse gas concentrations have rapidly decreased in the atmosphere.