Water quality tests show the presence of a single heavy metal in the drinking water pulled from a local river. Residents are suffering from numerous stomach ailments and are showing signs of long-term kidney damage. The residents of the next town, down river from the first town, seem to be unaffected. Water quality tests in the river by the second town show two types of heavy metals at acceptable legal limits in the water. One of the metals is the same as the one in the first town. What can scientists infer about the possible reason for the illnesses appearing in the first town but not the second?

One metal causes an acute effect, compared to the other.
One metal causes a chronic effect, compared to the other.
The heavy metals have a synergistic relationship when combined.
The heavy metals have an antagonistic relationship when combined.