The best prediction for the second part of Mark's experiment is:
The wagon will have more kinetic energy than when it was empty.
This is because kinetic energy is calculated using the formula \( KE = \frac{1}{2} mv^2 \), where \( m \) is mass and \( v \) is velocity. When the wagon has additional mass (from the bag of sand), the total mass increases, which results in an increase in kinetic energy, given that it is rolling down the hill and gaining velocity due to gravity.