What is a bone marrow?

Explanation

### Step-by-Step Solution:

1. **Definition of Bone Marrow**: Bone marrow is a semi-solid, spongy tissue found in the cavities of bones.

2. **Location**: It is primarily located in the cavities of long bones and is responsible for various important functions in the body.

3. **Function**: The main function of bone marrow is hematopoiesis, which is the production of new blood cells. This includes red blood cells (RBCs), white blood cells (WBCs), and other immune cells.

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