Which of the following statements is/are correct?

a. Carbon is an essential constituent of different organic compounds, such as amino acids, nucleic acids, proteins, vitamins and pigments. b. Only a few types of organisms, like certain species of soil bacteria and blue green algae, are capable of utilising carbon directly in its gaseous form. c. Carbohydrates that are decomposed by microorganisms, then get oxidised into carbon dioxide and are returned to the atmosphere.