What happens in plants during vascularisation?

• A. Differentiation of procambium, formation of primary phloem followed by formation of primary xylem.
• B. Differentiation of procambium followed by the formation of primary phloem and xylem simultaneously.
• C. Formation of procambium, primary phloem and xylem simultaneously.
• D. Differentiation of procambium followed by the formation of secondary xylem.