Mark the pair of substances among the following which is essential for coagulation of blood.

Options

A. Heparin and calcium ions
B. Calcium ions and platelet factors
C. Oxalates and cirtrates
D. Platelet factors and heparin

Explanation

To determine the pair of substances essential for the coagulation of blood, we need to analyze the options provided and understand the role of each substance in the blood clotting process.

### Step-by-Step Solution:

1. **Understanding Blood Coagulation**: Blood coagulation, or clotting, is a complex process that prevents excessive bleeding when there is an injury. It involves several components, including platelets and various clotting factors.

2. **Identifying Key Components**: The main components involved in blood coagulation are:
- **Platelets**: These are small cell fragments that play a crucial role in forming the initial plug at the site of injury.
- **Calcium Ions (Ca²⁺)**: Calcium ions are essential for the activation of several clotting factors that are necessary for the coagulation cascade.

3. **Evaluating the Options**:
- **Heparin and Calcium Ion**: Heparin is an anticoagulant and does not promote coagulation.
- **Calcium Ion and Platelet**: Both of these are essential for coagulation.
- **Oxalates and Citrates**: These are anticoagulants and inhibit clotting.
- **Platelet Factors and Heparin**: Again, heparin does not promote coagulation.

4. **Conclusion**: The pair of substances that are essential for the coagulation of blood is **Calcium Ion and Platelet**.

### Final Answer:
**Calcium Ion and Platelet**

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