The correct option is **D. I, II, III, and IV**. ### Explanation of the Answer Calcium carbonate (CaCO₃) is a common substance found in various natural forms. Let's break down each of the options provided to understand why all of them are indeed forms of calcium carbonate. 1. **Limestone**: - Limestone is a sedimentary rock primarily composed of calcium carbonate. It forms from the accumulation of marine organisms' shells, coral, and other calcium-rich materials. The chemical composition of limestone is predominantly CaCO₃, making it a direct form of calcium carbonate. 2. **Marble**: - Marble is a metamorphic rock that originates from limestone. When limestone undergoes heat and pressure, it recrystallizes to form marble, which is also composed mainly of calcium carbonate. The transformation process enhances the crystalline structure of the calcium carbonate, but it remains chemically the same (CaCO₃). 3. **Chalk**: - Chalk is a soft, white, porous form of limestone that is primarily composed of calcium carbonate. It is formed from the accumulation of microscopic marine organisms' shells, particularly coccolithophores. Like limestone, chalk is also a direct form of calcium carbonate. 4. **Egg-shells**: - Eggshells are primarily made of calcium carbonate as well. The eggshell consists of about 95% calcium carbonate, which provides strength and structure to the eggs. This makes eggshells another natural form of calcium carbonate. ### Why Other Options Are Incorrect or Weaker - **Option A (I only)**: This option is incorrect because it only includes limestone, ignoring the other forms of calcium carbonate present in the list (marble, chalk, and eggshells). - **Option B (I, II and IV)**: This option is also incorrect because it excludes chalk (III), which is a significant form of calcium carbonate. By omitting chalk, this option fails to recognize all the natural forms of calcium carbonate listed. - **Option C (I and II only)**: This option is incorrect as it only includes limestone and marble, neglecting both chalk and eggshells. It does not account for the complete range of calcium carbonate forms mentioned in the question. ### Summary of Key Points - **Calcium Carbonate Forms**: All four options (limestone, marble, chalk, and eggshells) are forms of calcium carbonate (CaCO₃). - **Limestone**: A sedimentary rock composed mainly of CaCO₃. - **Marble**: A metamorphic rock derived from limestone, retaining the same chemical composition. - **Chalk**: A soft, porous form of limestone, also primarily CaCO₃. - **Eggshells**: Composed of approximately 95% calcium carbonate, providing structural integrity. ### Revision Summary - Calcium carbonate (CaCO₃) exists in various natural forms. - Limestone, marble, chalk, and eggshells are all forms of calcium carbonate. - Each form has unique properties but shares the same chemical composition. - Understanding the different forms of calcium carbonate is essential in geology and biology.