Materials such as sucrose and amino acids can be transported in opposite directions through the phloem sieve tube elements. Which statement explains this?
- AActive transport occurs in some phloem sieve tube elements and mass flow occurs in other sieve tube elements.
- BBoth active transport and mass flow occur in each individual phloem sieve tube element.
- CMass flow occurs in both directions at the same time in each individual phloem sieve tube element.
- DMass flow occurs in different directions in different phloem sieve tube elements at the same time.