Aphids Are Small Insects That Insert Their Mouthparts into a Plant

Aphids are small insects that insert their mouthparts into a plant to obtain phloem sap. Which would be the best way to determine if aphids must actively draw phloem sap into their digestive tract or if hydrostatic pressure in the phloem tube could force the sap into them?

A) Isolate a phloem tube from a plant, allow an aphid to insert its mouthparts, and see if the aphid can still take up sap from it.
B) Measure relative rates of sugar manufacture in leaves with and without aphids.
C) Insert mouthparts removed from an aphid, without including the digestive tract, into phloem sap and see if sap keeps flowing through them.