Hello. You did not enter the text to which this question refers, which makes it impossible for it to be answered. However, I will try to help you in the best possible way.
The main idea of a text is the main subject that the text addresses, the subject on which all the arguments presented are based. You will be able to understand this idea by reading the full text and identifying what the text is about. In this case, the key sentence of the text is the sentence that presents this subject in the most direct way possible. Generally, the main idea and the key sentence of the text are presented in the first paragraphs.
To develop the main idea, the text presents details, which are sentences that support this idea, showing how it is relevant and why it is true. These details are present throughout the text.
Pollution interferes with the functioning of body systems, allowing malefic substances to come into contact with the body and be absorbed, acting in the body in a harmful way, fighting health.
It should be moved to the beginning of the sentence, that means, that it should be placed before "The motorcycles raced" (option A).
Explanation: "down the track" is a prepositional phrase. All prepositional phrases have no verbs nor subjects. These can be used as adjectives or adverbs. They can be placed either after the verb and at the begining of a sentence. When placed at the beginning of a sentence, it sounds more formal than the common structure composed of 'subject + verb + predicate' and this construction has to be followed by a comma.
In this case, the answer would be:
'Down the track, the motorcycles raced for the prize'.
Herbert Hover
Franklin Delano
Harry S. Truman
