According to paragraph 2, what is a problem for the explanation that life began in the oceans? A. Some life-forms would have been killed by high concentrations of H2CO and HCN in the early Earth's oceans. B. If complex organic compounds formed in Earth's oceans, they have been broken down by photochemical and thermal reaction. C. The essential elements for complex organic compounds did not exist on the early Earth. D. The creation of complex organic compounds on Earth's oceans was influenced by the photochemical and thermal reactions.