A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of heart attacks. It gives a broad overview of what causes heart attacks without going into details. This follows the rule of Topic Sentence. Why other lines are not chosen? The other paragraphs are not chosen because they do not stand alone. For example, 'That is called a clot' starts with a pronoun and needs context. 'It stops the blood flow' is also a continuation of an idea. 'Because of this reason the heart doesn’t pump the blood' is a result of previous information. All these sentences are dependent on earlier context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the idea of heart attacks. It shows the final effect of the previous information. This follows the Conclusive Words rule, as it summarizes the consequences of the earlier points.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains what the biological material is. This follows the Broader to Specific Strategy. The first paragraph gives a general idea, and the second gives a specific term for it.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it explains the effect of the clot. This follows the Cause and Effect Relationship rule. The second paragraph introduces the clot, and the third explains what happens because of it.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it shows the result of stopping blood flow. This follows the Cause and Effect Relationship rule. The third paragraph explains the effect, and the fourth explains the consequence.