The three main factors that determine the rate of diffusion across a membrane are related by the expression shown below. The rate of diffusion is proportional to $\frac{\text{surface area} \times \text{concentration difference}}{\text{thickness of membrane}}$. Which change or changes in these factors would make the rate of diffusion twice as large?
- A1, 2 and 4
- B1 and 3
- C1 and 4 only
- D2 and 3