Final answer:
When mp is negative, tp (temperature) falls.
Step-by-step explanation:
When mp is negative, tp (temperature) falls. This is because when a substance has a negative melting point (mp), it means that it is solid at temperatures below that point and will begin to melt when the temperature rises above that point. Therefore, as the temperature decreases and becomes negative, the substance will remain in a solid state, resulting in a decrease in temperature.