Which of the following statements about Ecosystem is / are correct ? (a) It is entirely a biological entity (b) It is an integrated unit (c) It is a study of interaction between physical environment and biological components (d) It is a study of stuctureal and functional aspects

• A. Only (a)
• B. (b) and (c)
• C. (b), (c) and (d)
• D. All the above