Which airway milestone is reached by week 16?

1. Bronchial buds branch until all segmental bronchi have formed. 2. Around week 13 the lumens of the bronchi begin to expand in diameter. 3. By week 16 respiratory bronchioles form and the fetus has all major lung structures involved in the airway. 4. Adequate surfactant and unaided breathing come much later, so the third and fourth options belong to the third trimester. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 22 "The Respiratory System", section 22.7 Embryonic Development of the Respiratory System_