Over the past 200 years the level of carbon dioxide in the atmosphere has
a. decreased slightly
b. increased until recently, then decreased
c. increased steadily
d. stayed aboutbthe same

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

answer:

convection currents in the magma drive plate tectonics. heat generated from the radioactive decay of elements deep in the interior of the earth creates magma (molten rock) in the aesthenosphere. the aesthenosphere (70 ~ 250 km) is part of the mantle, the middle sphere of the earth that extends to 2900 km.