Which statement(s) is/are correct for gas chromatography (GC)?

1. A gas chromatogram displays retention time on the $x$-axis and detector response on the $y$-axis.
2. The area under each peak on a gas chromatogram is proportional to the concentration of that component in the sample.
3. A mass spectrometer can be coupled with a gas chromatograph to identify the structures of the separated components.