______ the sun set behind the canyon walls, the temperature dropped rapidly in the desert air.
Which choice completes the text so that it conforms to the conventions of Standard English?
- A
Therefore
- B
And
- C
Then
- Dcheck_circle
As
Explanation
"As" introduces a dependent clause showing simultaneity, properly preceding the main clause with a comma. The others are coordinating conjunctions or adverbs requiring different structures.