Through studies on the Malpighian tubules, researchers found that K⁺ accumulated on the inner face of the tubule, against its concentration gradient. What can you infer about the mechanism of transport?

A) Movement of potassium into the lumen of the Malpighian tubules is an energy- requiring process.
B) Sodium ions will follow potassium ions.
C) Potassium transport is a passive process.
D) Potassium moves out of the tubules at a faster rate than it moves into the lumen of the tubules.