A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of color television systems. It does not refer to any previous information and gives a broad overview of the subject. 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, Paragraph 2 starts with "In the first color system," which needs context. Paragraph 3 uses "the receiving unit," which refers back to something not yet mentioned. Paragraph 4 starts with "This method," which also needs prior context.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the discussion about color television systems. It explains the method in detail and gives a complete view of the topic. This follows the rule of Summary Style.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it gives a specific example of one of the systems mentioned in Paragraph 1. This follows the Broader to Specific Strategy.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it explains what happens after the color system is used. It shows the result of the system mentioned in Paragraph 2. This follows the Logical Order.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it describes a method related to the monochrome images mentioned in Paragraph 3. This follows the Topic Example Strategy.