How many monochloro derivatives (structural isomers) can be obtained from 2,2-dimethylpropane by substitution of one hydrogen?

All twelve hydrogens of 2,2-dimethylpropane, $\mathrm{C}(\mathrm{CH}{}_3){}_4$, lie on four equivalent methyl groups, so only one product exists: 1-chloro-2,2-dimethylpropane (1).