The external parasites such as _______ affect the stomach of animals.

Options

A. worms
B. flukes
C. virus
D. bacteria

Explanation

**Step-by-Step Solution:**

1. **Identify the Type of Parasite**: The question asks for an external parasite that affects the stomach of animals. First, we need to understand what external parasites are. External parasites live on the surface of the host's body.

2. **Consider Common External Parasites**: Common external parasites include lice, fleas, ticks, and worms. However, we need to focus on those that specifically affect the stomach.

3. **Focus on Worms**: Among the external parasites, worms such as Ascaris are known to affect the stomach and intestines of animals. Although they are often considered internal parasites, they can be transmitted through external means (contaminated food or water).

4. **Understand the Impact of Ascaris**: Ascaris worms enter the host's body through ingestion of contaminated food. Once inside, they reside in the stomach and intestines, absorbing nutrients and causing malnutrition in the host.

5. **Eliminate Other Options**: Other mentioned parasites like liver flukes are internal parasites and do not fit the criteria of being external. Viruses and bacteria also do not qualify as external parasites affecting the stomach.

6. **Conclusion**: Based on the analysis, the correct answer to fill in the blank is "worms," specifically referring to Ascaris.

**Final Answer**: The external parasites such as **worms** affect the stomach of animals.

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