When people's symptoms of psychological distress are at their worst, whatever they do to try to alleviate the condition is likely to be followed by improvement rather than further deterioration. This is best explained in terms of

The answer is "regression toward the mean".

Explanation:

Regression towards the normal distribution is also known in statistical data as median regression, reversal to mean and reversal to mediocrity is the concept that emerges whenever a sample point of the random variable is almost an outliers, a futuristic point probably is closer to the center or average. Even before symptoms of depressive symptoms are by far the worst, anything they do to try and alleviate this same condition is more likely than worsening to improve. This is better illustrated by the return to the mean.

answer; /// i believe the correct answer is (when one group has an extremely high score at pretest;

portugal is the westernmost country in europe.