Consider the following statements: Which of the statements are correct?

• Statement I: One atomic mass unit (u) is defined as exactly one-twelfth of the mass of one carbon-12 atom.
• Statement II: The molar mass of a substance in g mol⁻¹ is numerically equal to its molecular (or formula) mass in u.
• Statement III: One mole of O₂ gas contains 6.022 × 10²³ oxygen atoms.
• Statement IV: The atomic mass of chlorine is 35.5 u because every chlorine atom has a mass of 35.5 u.