Different substances, such as sucrose and amino acids, may travel in opposite directions within the phloem sieve tubes. Which statement explains this?
- AActive transport occurs in some phloem sieve tubes and mass flow occurs in other phloem sieve tubes.
- BBoth active transport and mass flow occur in each individual phloem sieve tube.
- CMass flow occurs in both directions at the same time in each individual phloem sieve tube.
- DMass flow occurs in different directions in different phloem sieve tubes at the same time.