For the first question the answer is most likely A, because atmospheric CO2 has nothing to do with the convection currents in the mantle, or how often earthquakes occur or volcanoes erupt. CO2 dissolved in water forms carbonic acid, which erodes rock,and may cause caves to form
As for the second question, either By or DVD may be right. Crops are rotated to ensure that the soil does not get depleted of certain nutritients that some plants take up more than others. Legumes contain nitrogen fixing micro-organisms in their roots which replenish nitrogen into the soil.
