Final answer:
In the experiment to test which thermos keeps water the coolest, the control is the initial temperature of water, and the variable is the type of thermos. Option c is correct, establishing a consistent starting point for the comparison of the thermoses' insulating properties.
Step-by-step explanation:
When testing which thermos keeps water the coolest, the correct identification of the control and variables is crucial. The control is what is kept constant to ensure that the test conditions are stable, while the variables are what change to measure their effects. In this case, the control should be the water temperature at the beginning of the experiment, ensuring that each thermos starts with water at the same controlled temperature. The variables include the type of thermos, as you are comparing different thermoses to see how effective they are at keeping the water cool.
Therefore, the correct answer is:
c. Control: The temperature of water; Variables: The type of thermos.
By keeping the water temperature constant across all thermoses, any difference in the final temperature of the water after a set period of time can be attributed to the effectiveness of the different types of thermoses, making the type of thermos the independent variable. The dependent variable would be the final temperature of the water.