TOEFL Reading: ETS-TOEFL阅读机经 - _75M19MB6387XAQ5L$

According to paragraph 6, in addition to Milankovitch cycles, each of the following has been proposed as a contributing cause of ice ages EXCEPT A. variations in the composition of the Greenland ice cap B. variations in the Sun's energy output C. volcanic dust injected into the atmosphere D. decreases in atmospheric carbon dioxide