15. Which of the following does the heart belong? a. Organs c. Tissue b. Organ system d. Cell 16. What do you call a group freely interacting and breeding organisms of the same species? a. Population c. Biosphere b. Community d. Ecosystem 17. Which of these controls all the activities of the cell? a. Lysosome c. Nucleus b. Golgi body d. Mitochondria 18. Which of the following acts as a "power house of the cell? a. Mitochondria b. Golgi apparatus C. Ribosome d. Cell wall