Algal bloom cause
a. fish mortality.
b. deterioration of water quality.
c. increased oxygen in water body.
d. control of water pollution.

Options

A. a and c
B. b and d
C. a and b
D. b and c

Answer

To solve the question about the causes of algal bloom, we will analyze each option provided:

1. **Understanding Algal Bloom**: Algal blooms are rapid increases in the population of algae in water bodies, often due to nutrient enrichment, particularly nitrogen and phosphorus. This process is closely related to eutrophication.

2. **Analyzing Each Option**:
- **a. Fish mortality**: Algal blooms can lead to decreased oxygen levels in the water as decomposers break down the increased organic matter. This can cause fish mortality due to lack of oxygen. **(This option is correct.)**

- **b. Deterioration of water quality**: The presence of algal blooms can lead to poor water quality due to increased organic matter and reduced oxygen levels, which can harm aquatic life. **(This option is correct.)**

- **c. Increased oxygen in water body**: Algal blooms do not increase oxygen levels; in fact, they can lead to decreased oxygen levels as decomposition occurs. **(This option is incorrect.)**

- **d. Control of water pollution**: Algal blooms do not control water pollution; they often exacerbate it by consuming oxygen and degrading water quality. **(This option is incorrect.)**

3. **Conclusion**: The correct answers are **a (fish mortality)** and **b (deterioration of water quality)**.

### Final Answer:
The correct options are **a. fish mortality** and **b. deterioration of water quality**.