Thanks for the question! So remember what the principle underlying the psychologist’s argument is. It’s basically that sometimes, trying really hard to do something makes it harder, so it’s best to change focus to something else, and the thing we originally wanted might come.
Now take a look at (B), which tells us that the best way to succeed in writing a long document isn’t to think about how much is left to write, but only about the current paragraph, since thinking about what’s left will be super discouraging and might make the author ditch the project. This isn’t quite the same. The principle here is focusing on one aspect of a project instead of another to avoid discouragement. The principle was about shifting focus from one thing to a complete different thing (as opposed to two aspects of the same thing) because focusing on the first thing makes our goal harder to achieve. And that’s why (D) is a better choice than (B), and why (B) is incorrect.
Hope this helps! Feel free to ask any other questions that you might have.