## **Core Concept**
The working time of Zinc Phosphate (ZnPO4) cement is crucial in dental procedures, as it affects the manipulation and setting of the cement. ZnPO4 cement is commonly used for luting crowns and bridges. Its working time can be modified by various methods.

## **Why the Correct Answer is Right**
The correct answer involves understanding the properties and setting reactions of ZnPO4 cement. The setting reaction of ZnPO4 cement involves an acid-base reaction between zinc oxide powder and phosphoric acid liquid, producing zinc phosphate. Increasing the working time without compromising the final properties of the cement is essential.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Using a higher powder-liquid ratio can actually decrease the working time because it increases the viscosity of the mix more rapidly, making it difficult to work with.
- **Option B:** Cooling the mixing surface or using a cooled mixing slab can slow down the chemical reaction, thus increasing the working time. This method does not compromise the physical properties of the cement.
- **Option C:** Using a smaller powder-liquid ratio (more liquid) can increase the working time because it results in a less viscous mix initially, allowing more time for mixing and placement. However, it might affect the final strength.
- **Option D:** Adding a buffer can slow down the acid-base reaction, thereby increasing the working time.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **cooling the mixing slab** is a preferred method to increase the working time of ZnPO4 cement. This method physically slows down the reaction without altering the chemical composition or final properties of the cement.

## **Correct Answer: C.**