In this context, "billow" means to produce large quantities of smoke and dust, which could have the described catastrophic effects on the environment.

In this sentence, a "norm" is described as a "standard." This pairing of words makes sense and creates a coherent sentence. Norms often refer to accepted standards or behaviors within a group or society.

In this sentence, a "speedy" response is described as one made with "alacrity," which means briskness, eagerness, or willingness to respond quickly and with enthusiasm. This choice of words creates a logical and coherent sentence.

The sentence is comparing the difficulty of writing about two different things. The first part of the sentence sets up the contrast by stating that it is more difficult to write about something "surprisingly enough" than something else. The key to understanding the correct answer is to recognize the contrast between the two words in each pair. "Commonplace" refers to things that are ordinary, everyday, or familiar. These are things that are not particularly interesting or unusual. "Exotic" refers to things that are foreign, unusual, or unique. These are things that are out of the ordinary and intriguing because of their distinctiveness. Given this contrast, it makes sense that it would be more difficult to write about something commonplace (which lacks inherent interest or novelty) compared to something exotic (which has a built-in level of intrigue and uniqueness). Therefore, option C is the correct answer because it correctly captures this contrast in difficulty.