This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
Hydrocolloids are dimensionally unstable, they should be poured immediately. To avoid this, they should be stored at: 
Select and explain. 
A. 100% relative humidity, B. 2% potassium sulfate solution, C. Both of the above, D. None of the above
C: Both of the above
Answer: C: Both of the above 
Explanation: Once the hydrocolloid impression is removed from the mouth and exposed to air at room temperature, some shrinkage associated with syneresis and evaporation will occur. Conversely, if the impression is immersed in water, swelling caused by imbibition can occur. The storage medium, such as 2% potassium sulfate solution, or 100% relative humidity in a storage chamber, is suggested to reduce the dimensional change of agar impressions. 100% relative humidity is the best storage environment to preserve the normal water content of the impression.