Which is the best way to revise the underlined portion of sentence 12? Conservationists often say 'the last crop is always asphalt' to make the point that, when open lands are developed, they are gone for good.

Using em dashes is inappropriate here, as they suggest an abrupt break in thought or an emphasis that doesn't fit the context. The phrase "when open lands are developed" is a subordinate clause that shouldn't be emphasized in this manner, leading to a disruption in the flow of the sentence.

Parentheses imply that the information within them is supplementary or an afterthought, which could undermine the importance of the clause. The clause "when open lands are developed" is crucial for conveying the intended meaning and should be integrated more seamlessly into the sentence with a comma instead.

This option lacks a comma after "that," which is necessary to clearly delineate the main clause from the subordinate clause. Omitting the comma can lead to confusion in understanding the relationship between the two parts of the sentence, making it less effective.