The first formed primary phloem is referred to as $A$. The later formed primary phloem is referred to as $B$. Choose the correct combination of $A$ and $B$.

• A
  $A$-protoxylem,$B$-metaxylem
• B
  $A$-protophloem,$B$-sieve tube cells
• C
  $A$-metaphloem,$B$-sieve tube cells
• D
  $A$-protophloem,$B$-metaphloem