Atmospheric Carbon Dioxide Levels Are Shown Below

Atmospheric carbon dioxide levels are shown below.This graph was developed by measurements that were obtained in which of the following two ways?

A) direct measurements on Mauna Loa, Hawaii, and from air bubbles trapped in ice.
B) studies of plant structures in response to differing CO₂ levels and measurements of carbon dioxide within rock layers.
C) carbon isotope levels in seafloor sediments and from air bubbles trapped in ice.
D) measurements from undisturbed bat droppings in caves on a South Pacific island and carbon isotope levels in sea floor sediments.