Secretion and absorption are the main functions  

Options

A. pavement epithelium of vein
B. tall and slender cells of intestine
C. cuboidal cells of ducts of glands & tubes of nephrons
D. both B and C

Answer

To answer the question "Secretion and absorption are the main functions of what?", we can analyze the given options step by step.

### Step-by-Step Solution:

1. **Understanding the Options**:
- The options provided are:
- A) Pavement epithelium of veins
- B) Tall and slender cells of the intestine
- C) Cuboidal cells of ducts of glands and tubes of nephron

2. **Analyzing Option A - Pavement Epithelium of Veins**:
- The pavement epithelium is a type of simple squamous epithelium.
- It is primarily found in the lining of blood vessels, including veins.
- Its main function is to act as a barrier and minimize diffusion between blood and surrounding tissues.
- **Conclusion**: This option does not perform secretion or absorption. Therefore, it is **incorrect**.

3. **Analyzing Option B - Tall and Slender Cells of the Intestine**:
- These cells refer to columnar epithelial cells found in the intestinal lining.
- They have microvilli on their surface, which increase the surface area for absorption.
- They are involved in the secretion of digestive enzymes and absorption of nutrients from digested food.
- **Conclusion**: This option is involved in both secretion and absorption. Therefore, it is **correct**.

4. **Analyzing Option C - Cuboidal Cells of Ducts of Glands and Tubes of Nephron**:
- Cuboidal epithelial cells are found in gland ducts and nephron tubules in the kidneys.
- They also have microvilli that assist in secretion and absorption processes.
- Their functions include secretion of substances and reabsorption of water and solutes in the nephron.
- **Conclusion**: This option is also involved in both secretion and absorption. Therefore, it is **correct**.

5. **Final Conclusion**:
- Since both options B (tall and slender cells of the intestine) and C (cuboidal cells of ducts of glands and tubes of nephron) are involved in secretion and absorption, the correct answer is **both B and C**.