Let $\text{R = (A, B, C, D, E, F)}$ be a relation scheme with the following dependencies $\text{C} \rightarrow \text{F, E} \rightarrow \text{A}, \text{EC} \rightarrow \text{D, A} \rightarrow \text{B}$. Which of the following is a key of $\text{R}?$
- $\text{CD}$
- $\text{EC}$
- $\text{AE}$
- $\text{AC}$