(B) This sentence correctly pairs the elements in the "both . . . and" construction: both "to substitute [something] and to enhance [something]." (Note: "to both substitute . . . and to enhance" would be wrong. Why? But "to both substitute and enhance" would be right. Why?)
(A) Playing by ear, a reader should find this option awkward. The preposition "for" is separated from its object. The shift from the verb "will be" to "should be" makes no sense. The "should be" strands the reader in a murky world of "fertility" that might happen? or ought to happen?
(C) Again, note the awkward use of "for". And also note: "that" replaces the use of "so that" in item A. "So that" is better. It properly introduces a reason or an explanation.
(By the way, "that" functions properly in the following kind of sentence: "The reason for his success is that (not "because") he studied regularly.")
(D) "so as" in "so as to substitute" is redundant and substandard. "To substitute . . ." fully expresses one reason why farmers apply more fertilizer.
"To substitute" followed by "and" calls for another infinitive (to + verb). Imagine the two suspended parts of a scale properly balanced on either side of a fulcrum. But here the expected balance or parallel construction doesn't follow.
What does follow shifts the meaning of the sentence. Farmers apply fertilizer for two reasons: to substitute for eroded topsoil and to enhance fertility. The words "and fertility will be enhanced" suggest that this enhancement is additional or secondary.
(E) Again, coordinate ideas should be expressed by identical constructions: "to substitute' for eroded topsoil and to enhance' fertility."
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