Answer: The version of the underlined text that best maintains the tone of the passage is B. wholly necessary to show the difference best maintains the tone of the passage.
Explanation: The tone of the passage is informative and objective, and "wholly necessary to show the difference" fits this tone because it emphasizes the importance of understanding the distinction between the two concepts. "Disturbing to note the marked contrast" would imply a negative connotation and a more emotional tone, which does not match the rest of the passage.