The correct option to complete the sentence "The train rattled along .... full speed" is B. at. Explanation of the Correct Answer

1. Understanding the Phrase: The phrase "at full speed" is a common English expression that indicates something is moving as fast as it can. In this context, "full speed" refers to the maximum velocity of the train.
2. Preposition Usage: The preposition "at" is used to denote a specific point or condition. When we say "at full speed," we are specifying the condition of the train's movement. It implies that the train is currently operating at its maximum speed.
3. Contextual Fit: The sentence describes the action of the train. Using "at" fits naturally in the context of describing how fast the train is moving. It conveys the idea that the train is not just moving, but doing so at its highest possible speed.

Why the Other Options Are Incorrect

• A. in: The preposition "in" is typically used to indicate being inside something or within a certain condition. For example, "in a hurry" or "in the car." Saying "in full speed" is not idiomatic in English and does not convey the intended meaning of the train's speed.
• C. with: The preposition "with" suggests accompaniment or possession. For instance, "with great enthusiasm" or "with a friend." Saying "with full speed" is also not standard English usage. It does not effectively communicate the idea of the train's speed.
• D. by: The preposition "by" can indicate proximity or the means of doing something. For example, "by train" or "by doing this." However, "by full speed" does not make sense in this context. It does not convey the idea of the train's speed effectively.

Summary of Key Points

• The correct answer is B. at, as it is the appropriate preposition to describe the condition of the train's speed.
• "At full speed" is a common expression indicating maximum velocity.
• The other options (A, C, D) do not fit the context or are not idiomatic in English.
• Understanding the correct use of prepositions is crucial for constructing meaningful and grammatically correct sentences.

By focusing on the context and the common expressions used in English, you can improve your understanding of how to choose the right words in similar situations.