A person with type O blood is married to a person with type A blood (unknown genotype). They have 6 children, 3 of them have type A blood, three of them have type O blood. What is the genotype of the two parents?

The large increase in atmospheric carbon dioxide in the last 50 years most likely comes from a. an increase in cellular respiration b. increased decomposition by bacteria c. an increase in the burning of fossil fuels d. an increase in photosynthesis