How Can Researchers Confirm That Phloem Moves from a Source

How can researchers confirm that phloem moves from a source to a sink?

A) They can observe aphids and cut off their beaks.
B) They can study the cause of maple sap flow in producing syrup.
C) They can observe the effects of girdling a tree below the level of the majority of leaves.
D) They can use radioactive tracers to measure the flow of sap between two distances over time.
E) They can observe the uptake of ink into the stem of celery.