[单选题]
The diagram shows sucrose being passively l6-aouu t6 z3hqoaded into the sieve tube f y;erahi-,l .unaided, before being transporteyf .;-ae,rh lid around the plant.
Which structure is labelled X?
A. Plasmodesmata
B. Proton pump
C. Co-transporter protein
D. Stomata