Final answer:
In an experiment on the effect of global temperature on ocean levels, the average ocean level over the last 100 years serves as the control, providing a stable reference for comparison.
Step-by-step explanation:
The average ocean level over the last 100 years in an experiment measuring the effect of global temperature increase on ocean levels would be considered d. the control.
In experimental design, a control is a standard or reference point for comparison that remains constant and unchanged throughout the experiment. The control is used to understand what happens when a variable is manipulated.
In this case, the historical average ocean level provides a baseline against which the present ocean levels under the condition of increased global temperature can be compared. This comparison helps to isolate the impact of the global temperature increase on ocean levels by providing a reference that excludes recent changes or potential influencing factors.
It is crucial to note that other factors such as thermal expansion and melting of ice caps contribute to rising ocean levels. Thermal expansion occurs when water increases in volume and decreases in density as it warms, which is a significant factor in sea level rise due to climate change. This effect is mentioned to underscore the complexity of the phenomenon since the ocean does not warm uniformly with depth, making the impact on ocean levels more intricate to calculate.