What represents 0 on the x-axis of a proton NMR spectra?

Question 2 options:

	The 0 value is established by starting the radio energy at 0
	A pure sample of the unknown is run through the NMR to establish a 0 reference before it is subjected to radio wave energy and magnetic fields
	The 0 value is established by setting the magnetic field to 0
	Tetramethylsilane is often used to establish the right-most peak