## Core Concept
The question involves understanding the principles of SDS-PAGE (Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis) and the role of mercaptoethanol in protein analysis. SDS-PAGE is a technique used to separate proteins based on their size (molecular weight). Mercaptoethanol is a reducing agent that breaks disulfide bonds in proteins.

## Why the Correct Answer is Right
The protein with a molecular weight of 100 kD shows two bands of 20 kD and 30 kD after treatment with mercaptoethanol. This indicates that the original 100 kD protein is composed of two subunits: one with a molecular weight of 20 kD and the other with a molecular weight of 30 kD. These subunits were likely linked by disulfide bonds, which were broken by mercaptoethanol, resulting in the separation into two distinct bands on the SDS-PAGE gel.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, so it cannot be evaluated.
- **Option B:** This option is not provided, so it cannot be evaluated.
- **Option C:** This option is not provided, so it cannot be evaluated.
- **Option D:** This option is not provided, but based on the context, if an option suggests that the protein is not composed of subunits or misinterprets the role of mercaptoethanol, it would be incorrect because mercaptoethanol's role in breaking disulfide bonds is correctly linked to the separation of protein subunits.

## Clinical Pearl / High-Yield Fact
A key point to remember is that mercaptoethanol (or beta-mercaptoethanol) is used to reduce disulfide bonds in proteins, which can lead to the dissociation of protein subunits. This is a critical step in protein analysis to determine the subunit composition of proteins.

## Correct Answer: C.