Why is the oscillator in phase with the driver when driving frequency is low?

[Daniel Wqw]

Homework Statement

Can someone please help me to make sense of this statement: When the driving frequency is much less than the natural frequency then the driver is in phase with the oscillator. When the driving frequency is much greater than the natural frequency the oscillator can't keep up so the oscillator and the driver are completely out of phase (anti-phase).

Homework Equations

The Attempt at a Solution

 

[Nidum]