Read the passage below and answer the question.

Ask a final-year student how long her project report will take, and she will usually give a confident answer: one weekend, perhaps two. Ask her again a month later, and the report is often still unfinished. This is not simple laziness. Psychologists call it the planning fallacy: we keep believing that the next task will go quickly, even when the earlier ones all ran late.

The error comes from the way we picture the work. While planning, we imagine the task itself, step by step, and we imagine every step going smoothly. We do not picture the laptop that will not start, the relative who visits without warning, or the chapter that has to be written twice. Each of these delays is unlikely on its own, but taken together, it is almost certain that at least one of them will happen. The error is also stubborn. People who are reminded of their past delays still make hopeful plans, because they treat those delays as special cases that will not happen again.

The remedy is simple. Instead of asking how long the work should take, ask how long similar work actually took last time. The record of such tasks, your own or other people's, usually gives a far more realistic estimate than the picture in your head. None of this makes a planner slow or gloomy. It only means that the plan rests on what usually happens rather than on what we hope will happen.

Question: Which of the following best states the main idea of the passage?