Final answer:
The sentence should read 'The sky is covered with thick rain clouds,' using the preposition 'with' to indicate that the sky is filled by the rain clouds.
Step-by-step explanation:
The correct preposition to use in the sentence 'The sky is covered___ thick rain clouds' is 'with'. Therefore, the complete sentence should read, 'The sky is covered with thick rain clouds.' The use of 'with' indicates that the sky is filled or blanketed by the rain clouds, which is the intended meaning in this context.
Descriptions of the sky and clouds often convey mood and setting, as evidenced by the provided excerpts. Imaginative language can paint vivid pictures of natural phenomena, such as the 'purple cloud-bank' described as obscuring the sun's 'fiery red heart' or the 'thundering gray clouds' from which rain pours down fiercely upon the pavement. Such literary depictions contribute to the atmosphere and tone of a narrative.