First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of the text, which is the central processing unit (CPU) of a computer. It gives a broad overview of what the CPU is and where it is found. 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 "The microprocessor" which needs context. Paragraph 3 starts with "It receives" which uses a pronoun that refers back to the microprocessor. Paragraph 4 starts with "The brain" but it is not clear without the first sentence.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the explanation of the microprocessor's functions. It summarizes the actions of the microprocessor, which follows the Summary Style rule.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the topic of the microprocessor. It uses the Connector strategy by starting with "The microprocessor" which directly relates to the previous paragraph.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it explains what the microprocessor does. It uses the Noun Pronoun Rule, as "It" refers back to the microprocessor mentioned in Paragraph 2.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it explains what happens after the microprocessor receives the signals. It follows the Logical Order strategy, showing the process of handling the codes.