Its C, The result of the "dear aunt jane" experiment.
Yes and no. But mostly yes.
The Mayans impressively developed a concept of zero as a number and as a placeholder. They did so in the 4th century AD, and what makes it especially impressive is that they more or less did so on their own, without borrowing from or exchanging ideas with other civilizations.
However, unlike ancient India, who developed the concept of zero around the same time, the Maya never applied to to mathematical calculations, using it mostly as a placeholder and a concept in their complex calenders.
Zero really becomes a powerful force in science and math when you begin applying the concept to equations. A number times zero is zero, a number divided by zero can't exist but theoretically is infinite, etc. Although the Mayans never applied zero to their impressive advancements in geometry, construction, etc, they did invent the concept essentially from scratch, which is an impressive feat that few other societies accomplished in the ancient world.
The confounding variable in this situation is the disparity between the days of the week on which individuals performed physical exercise to improve their mood.
Perhaps, individuals generally have a lower and more supressed mood on Mondays versus Thursdays causing the results to be skewed as they failed to account for the <em>confounding variable of different dates</em>.
Both can be plausible answers. One is a common psychological response to adversity and/or a simple social construct [brids of a feather flock together] that allows for like-minded people to band together.
On the other hand, the saying 'opposites attract' may apply to people who may be drawn into certain individuals because of traits that they lack firsthand. If someone is inquisitive and humorous while the other is serious and tactile, they could form a productive educational group or mold a fitting romantic relationship.
