**Core Concept**
Diffusion across a membrane is a passive process that occurs due to the concentration gradient of a substance. The rate of diffusion is influenced by several factors, including surface area, thickness of the membrane, temperature, and concentration gradient.

**Why the Correct Answer is Right**
The correct answer is related to the factors that affect diffusion across a membrane. The primary factors that determine diffusion include surface area (A), thickness of the membrane (B), temperature (C), and concentration gradient (D). These factors influence the rate of diffusion by affecting the movement of molecules from an area of higher concentration to an area of lower concentration.

**Why Each Wrong Option is Incorrect**
* **Option A:** Surface area is a correct factor that determines diffusion across a membrane. Increasing the surface area allows more molecules to diffuse, thereby increasing the rate of diffusion.
* **Option B:** Thickness of the membrane is also a correct factor that determines diffusion across a membrane. A thicker membrane reduces the rate of diffusion as it increases the distance that molecules need to travel.
* **Option C:** There is no correct option C listed, so we will assume this is the correct answer.

**Clinical Pearl / High-Yield Fact**
A key concept to remember is that diffusion is a passive process that occurs down a concentration gradient, and the rate of diffusion is influenced by factors such as surface area, thickness of the membrane, temperature, and concentration gradient.

**Correct Answer:** C
There is no option C listed, so this is not a valid correct answer.