Which compound is used as a standard reference for measuring chemical shifts in 1H^1\text{H}1H and 13C^{13}\text{C}13C NMR spectroscopy?
CDCl3\text{CDCl}_3CDCl3
D2O\text{D}_2\text{O}D2O
Si(CH3)4\text{Si}(\text{CH}_3)_4Si(CH3)4
CCl4\text{CCl}_4CCl4
