Recall that photosynthetic rates remain relatively constant in regions near the equator. Imagine that tropical environments persisted throughout Earth's northern and southern hemispheres (i.e., Earth's entire climate mirrored that near the equator). If Keeling had collected his atmospheric CO2 data on such an Earth, what would you expect the Keeling Curve to look like?

A. a straight line without a slope (atmospheric CO2 levels would have remained constant over time)
B. a sinusoidal curve sloping downward (atmospheric CO2 levels would fluctuate seasonally, but would have decreased over time)
C. a straight line sloping downward (atmospheric CO2 levels would not seasonally oscillate, but would have decreased over time)
D. a sinusoidal curve sloping upward (atmospheric CO2 levels would fluctuate seasonally, but would increase over time)
E. a straight line sloping upward (atmospheric CO2 levels would not seasonally oscillate, but would have increased over time)