Final Answer:
The last word in the sentence "Because there are so many sources of carbon dioxide" is option 4) many.
Step-by-step explanation:
In the given sentence, "Because there are so many sources of carbon dioxide," the word that appears last in the sequence is "many." The sentence structure indicates a cause-and-effect relationship, explaining the reason behind the abundance of carbon dioxide. Each word plays a specific role in conveying the meaning, and "many" concludes the sentence by emphasizing the quantity or multitude of sources contributing to the presence of carbon dioxide.
Therefore, the answer is option 4) many.