In Spectroscopy, a Diffraction Grating Is A

In spectroscopy, a diffraction grating is a:

A) screen placed at the focus of a telescope to provide a calibrated grid on photographs.
B) source of light that provides calibration lines alongside the measured spectrum.
C) piece of glass or metal on which many closely spaced grooves have been cut.
D) prism of very pure glass through which light can pass.