Find the correct statement.

Options

A. The clean water organisms return in a river in which sewage has been discharged when the BOD is low.
B. Having an algal bloom in a lake indicats good quality of water.
C. According to CPCB, particulate size 25 micrometers or less in air cause the greatest harm to human health.
D. In DDT bio magnification the maximum ill effect is on the zooplanktons

Answer

To find the correct statement from the given options, let's analyze each statement step by step.

### Step 1: Analyze Statement 1
**Statement 1:** "The clean water organism written in river in which sewage has been discharged when the BOD is low."

- **Analysis:** This statement is incorrect. When sewage is discharged into a river, the Biological Oxygen Demand (BOD) is typically high due to the presence of organic matter. A low BOD indicates that the water is clean and has less organic pollution. Therefore, this statement is wrong.

### Step 2: Analyze Statement 2
**Statement 2:** "Having an algal bloom in lake indicates good quality of water."

- **Analysis:** This statement is also incorrect. An algal bloom is often a sign of nutrient pollution (like nitrogen and phosphorus) and can lead to poor water quality. Algal blooms can deplete oxygen in the water and produce toxins, indicating bad water quality. Therefore, this statement is wrong.

### Step 3: Analyze Statement 3
**Statement 3:** "According to CPCB, particulate size 25 micrometers or less in air cause the greatest harm to human health."

- **Analysis:** This statement is correct. The Central Pollution Control Board (CPCB) identifies particulate matter of size 25 micrometers or less as harmful to human health. These fine particles can penetrate deep into the lungs and even enter the bloodstream, causing various health issues. Therefore, this statement is correct.

### Step 4: Analyze Statement 4
**Statement 4:** "In DDT biomagnification, the maximum ill effect is on the zooplanktons."

- **Analysis:** This statement is incorrect. In the case of DDT biomagnification, the highest concentration of DDT is found in apex predators (top of the food chain), such as large fish and birds, rather than in zooplankton. Zooplankton may be affected, but they do not experience the maximum ill effects. Therefore, this statement is wrong.

### Conclusion
After analyzing all four statements, the only correct statement is **Statement 3**.

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