Cindy predicts that plastic foam insulates cold drinks better than metal or ceramic materials do. to test the hypothesis, she fills cups made from these materials with equal amounts of cold water. she records the temperature of the water in each cup, using scientific thermometers, every 10 minutes until the water reaches room temperature. which of these conditions must be the same for this experiment to be valid?
a. the thermometer that is in each cup
b. the starting temperature of the water in each cup
c. the ending temperature of the water in each cup
d. the material that makes up each cup

B. The starting temperature of the water in each cup  

Explanation:

Cindy is trying to see if foam or ceramic is a better insulator. Those are her independent variables.

The other variables , like the starting temperature of the water in each cup, must be controlled variables. If she uses different starting temperatures in each cup, she won't know if it was the temperature or the materials that caused her results.

A. is wrong. The thermometers should be identical but, if they aren't, it will make little difference in the results.

C. is wrong. The ending temperature is room temperature, so it is automatically the same for each cup.

D. is wrong. She is trying to measure the effect of different materials.

they are all solids.