Final answer:
When a fluid reaches a constant temperature, the driving force behind convection currents is lost, leading to their disappearance.
Step-by-step explanation:
Convection currents occur in fluids due to the movement of a fluid resulting from temperature differences within the fluid. These currents are responsible for transferring heat, with warmer, less dense material rising and cooler, denser material sinking. When a fluid reaches a constant temperature, there is no longer a temperature gradient to drive this movement, and as a result, the convection currents will disappear.