Identify the correct statements about blood.
A. Platelets are produced in the bone marrow.
B. When haemoglobin combines with oxygen it forms carboxyhaemoglobin.
C. Calcium ions play an important role in clotting of blood.
D. Fibrins are formed by the conversion of fibrino gen by the enzyme thrombin.

Options

A. A and B only
B. B, C and D only
C. B and D only
D. A, C and D only

Explanation

To solve the question of identifying the correct statements about blood, we will evaluate each statement one by one.

### Step-by-Step Solution:

1. **Evaluate Statement A**:
- "Platelets are produced in the bone marrow."
- Platelets, also known as thrombocytes, are indeed produced in the bone marrow from megakaryocytes. They play a crucial role in blood clotting.
- **Conclusion**: This statement is **correct**.

2. **Evaluate Statement B**:
- "When hemoglobin combines with oxygen it forms carboxyhemoglobin."
- Hemoglobin is a protein in red blood cells that binds with oxygen to form oxyhemoglobin. Carboxyhemoglobin is formed when hemoglobin binds with carbon monoxide, not oxygen.
- **Conclusion**: This statement is **incorrect**.

3. **Evaluate Statement C**:
- "Calcium ions play an important role in clotting of blood."
- Calcium ions are essential for the blood coagulation process. They help in the activation of several coagulation factors, which are necessary for blood clotting.
- **Conclusion**: This statement is **correct**.

4. **Evaluate Statement D**:
- "Fibrins are formed by the conversion of fibrinogen by the enzyme thrombin."
- This statement is accurate. During the clotting process, thrombin converts fibrinogen (a soluble plasma protein) into fibrin (an insoluble protein), which forms the mesh that constitutes the clot.
- **Conclusion**: This statement is **correct**.

### Final Evaluation:
- The correct statements about blood are:
- A. Platelets are produced in the bone marrow. (Correct)
- B. When hemoglobin combines with oxygen it forms carboxyhemoglobin. (Incorrect)
- C. Calcium ions play an important role in clotting of blood. (Correct)
- D. Fibrins are formed by the conversion of fibrinogen by the enzyme thrombin. (Correct)

Thus, the correct options are A, C, and D.