**Core Concept**
The discoloration of the cervical margin of a tooth after a dental restoration, such as a gold inlay fused with porcelain, can be attributed to various factors. This phenomenon highlights the importance of material interaction and chemical reactions in dental restorations. The specific discoloration observed is a result of the interaction between gold and other elements present in the porcelain.

**Why the Correct Answer is Right**
The presence of silver in the gold alloy is the primary cause of the greenish discoloration at the cervical margin. When gold fused with porcelain is fired at high temperatures, the silver content reacts with the oxygen present in the porcelain, leading to the formation of silver oxide. This silver oxide is responsible for the characteristic greenish coloration observed. The reaction between silver and oxygen is a well-known phenomenon in dental materials, and it is essential to consider this interaction when selecting materials for dental restorations.

**Why Each Wrong Option is Incorrect**
**Option A:** High firing temperature is not the primary cause of the greenish discoloration. While high temperatures can accelerate chemical reactions, they do not directly contribute to the discoloration.

**Option B:** Disintegration of the opaque layer is not directly related to the greenish discoloration. The opaque layer is a critical component of the porcelain, and its disintegration would lead to other complications, such as leakage or marginal breakdown.

**Option D:** Formation of excess oxides is a possible consequence of high firing temperatures, but it is not the primary cause of the greenish discoloration. Excess oxides can lead to other issues, such as discoloration or surface roughness, but they are not directly responsible for the greenish coloration observed.

**Clinical Pearl / High-Yield Fact**
It is essential to consider the chemical interactions between dental materials, particularly when selecting materials for restorations. The presence of certain elements, such as silver, can lead to unexpected discoloration or other complications. A thorough understanding of these interactions is critical for achieving successful dental restorations.

**✓ Correct Answer: C. Presence of silver**