When Glenn tries to move two overlapping circles, how can he separate their movements?

Using different layers for the shapes is the most effective way for Glenn to separate their movements when dealing with overlapping circles. Layering allows each shape to be manipulated independently without affecting the position of the other shapes on different layers. This gives Glenn the flexibility to move, edit, or animate each circle separately, thus maintaining control over their individual behaviors.

The process of merging or grouping shapes would combine them into a single entity, which would prevent independent movement. Similarly, resizing shapes doesn't address the issue of movement at all; it only alters their dimensions. Hence, layering provides the necessary separation that allows for distinct manipulation of overlapping shapes.