Explanation:

Let’s look at the sentences and options:

- D. Global warming is the single biggest threat to life on earth today.

* Best as an introductory/general statement.

- B. The term is used to describe the unnatural rise in earth’s average temperature.

* Explains what global warming is.

- C. Increased carbon dioxide is the primary driver of global warming.

* Explains the main cause.

- A. There are many secondary reasons as well along with increasing levels of carbon.

* Adds to the explanation by mentioning secondary reasons.

Option 2—DBCA:

- D (introduction) ? B (defines) ? C (main cause) ? A (other causes)

Other options:

- Option 1 (DABC): Switches the order of explanations; less logical because it states secondary reasons before primary.

- Option 3 (ABCD): Starts mid-thought, making it less coherent.

- Option 4 (ACBD): Does not create logical flow.

So, the correct order is DBCA.

Option 2: DBCA is the correct answer.