`Al_2(SO_4)_3` is used in the following but not

Options

A. as a coagulant in treating drinking water
B. in plastic industry
C. as a mordant in dyeing
D. both C and D

Answer

To determine where `Al_2(SO_4)_3` (aluminum sulfate) is used and where it is not, we will analyze the provided options one by one.

### Step-by-Step Solution:

1. **Identify the Uses of Aluminum Sulfate:**
- **Coagulating Agent in Water Treatment:** Aluminum sulfate is widely used as a coagulant in the treatment of drinking water. It helps in the aggregation of impurities, making them easier to remove.
- **Moderating Agent in Dyeing:** Aluminum sulfate is also used in the dyeing process as a mordant. It helps in fixing dyes onto fibers by forming aluminum hydroxide, which binds with the dye.
- **Plastic Industry:** There is no significant use of aluminum sulfate in the plastic industry.

2. **Evaluate Each Option:**
- **Option A:** Coagulating in treating drinking water - This is a correct use of aluminum sulfate.
- **Option B:** In plastic industry - This is NOT a correct use of aluminum sulfate.
- **Option C:** As a mordant in dyeing - This is a correct use of aluminum sulfate.
- **Option D:** Both C and D - This option cannot be correct since it implies that aluminum sulfate is used in the plastic industry, which is false.

3. **Conclusion:**
- Since aluminum sulfate is used in water treatment and dyeing, but not in the plastic industry, the correct answer to the question is **Option B: In plastic industry**.

### Final Answer:
`Al_2(SO_4)_3` (aluminum sulfate) is used in the following but not in the plastic industry.