To enter or leave a cell, the substance must pass through .........and its structure was explained by ....... in the year ...........

Options

A. `{:(P,Q,R),("the cell wall ","the cell theory ",1978):}`
B. `{:(P,Q,R),("the plasma membrane, ","the cell theory ",1942):}`
C. `{:(P,Q,R),("the cell wall ","the fluid mosaic model ",1975):}`
D. `{:(P,Q,R),("the plasma membrane, ","the fluid mosaic model ",1972):}`

Answer

To solve the question, we need to fill in the blanks regarding the entry and exit of substances in a cell and the model that explains the structure of the cell membrane.

### Step-by-Step Solution:

1. **Identify the Structure Through Which Substances Pass:**
- To enter or leave a cell, substances must pass through the **plasma membrane** (also known as the cell membrane). This membrane is crucial as it regulates what enters and exits the cell.

2. **Identify the Model Explaining the Structure:**
- The structure of the plasma membrane is explained by the **fluid mosaic model**. This model describes the membrane as a fluid combination of phospholipids, cholesterol, and proteins, which allows for flexibility and movement.

3. **Identify the Year of the Model's Introduction:**
- The fluid mosaic model was proposed in the year **1972**. This model was significant in understanding the dynamic nature of the cell membrane.

### Final Answers:
- To enter or leave a cell, the substance must pass through **plasma membrane**.
- Its structure was explained by **fluid mosaic model**.
- In the year **1972**.