**Core Concept**
Zinc phosphate cement is a traditional dental cement used for luting crowns and bridges. Its properties and handling require a specific technique to achieve optimal results.

**Why the Correct Answer is Right**
The correct answer involves understanding the mechanism of zinc phosphate cement setting. When zinc phosphate cement is mixed, it undergoes a complex reaction involving the dissolution of zinc oxide particles in phosphoric acid. To control the setting time and prevent premature setting, the cement is mixed on a cool glass slab, and a small amount of powder is added to the liquid every 20 seconds. This technique, known as "successive addition of powder," allows for a gradual release of heat and prevents the reaction from becoming too exothermic, which can lead to premature setting.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the context of zinc phosphate cement mixing.

**Option B:** While controlling the mixing time is crucial, the correct reason for adding powder every 20 seconds is to prevent premature setting due to heat generation.

**Option C:** The correct answer is not related to the working time of the cement.

**Option D:** This option is not relevant to the specific technique being used.

**Clinical Pearl / High-Yield Fact**
To remember the correct technique for mixing zinc phosphate cement, the phrase "cool glass, slow and steady, and successive additions" can be used. This phrase highlights the importance of mixing the cement on a cool glass slab, adding powder slowly, and making successive additions to control the setting time.

**Correct Answer:** C.