**Core Concept**
The strength of amalgam is a critical factor in determining its durability and resistance to wear and tear. The setting reaction of amalgam involves the conversion of gamma-2 (γ2) phase into gamma-1 (γ1) phase, leading to an increase in compressive strength.

**Why the Correct Answer is Right**
The strength of amalgam is attained through the setting reaction, which is a complex process involving the interaction of mercury with the alloy particles. The initial strength of amalgam is achieved within 8 hours, after which it continues to increase over time. The correct answer of 70% reflects the optimal setting reaction, which is crucial for the clinical success of amalgam restorations. The γ2 to γ1 phase conversion is facilitated by the interaction between the alloy particles and the mercury, leading to an increase in compressive strength.

**Why Each Wrong Option is Incorrect**
**Option A:** 60% is an underestimation of the initial strength of amalgam, as it does not accurately reflect the optimal setting reaction.
**Option C:** 80% overestimates the initial strength of amalgam, as it takes longer than 8 hours for the amalgam to reach its maximum compressive strength.
**Option D:** 90% is an unrealistic value for the initial strength of amalgam, as it exceeds the maximum compressive strength achievable through the setting reaction.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the strength of amalgam can be influenced by various factors, including the composition of the alloy, the type of mercury used, and the mixing technique employed. A well-mixed amalgam with the optimal ratio of alloy to mercury will exhibit the highest compressive strength.

**✓ Correct Answer: B. 70%**