On heating with chlorine, the hydrocarbon $2,2$-dimethylbutane undergoes free radical substitution. During a propagation step, a free radical $X\bullet$ is produced: $\text{CH}_3\text{CH}_2\text{C(CH}_3)_2\text{CH}_3 + \text{Cl}\bullet \rightarrow X\bullet + \text{HCl}$. How many different forms of $X\bullet$ can exist?
- A1
- B2
- C3
- D4