Problem - Minimum Arrows

Balloons are represented as intervals on the x-axis. An arrow at position $x$ bursts all balloons where $start \leq x \leq end$. Find minimum arrows to burst all balloons.

Example: [[10,16], [2,8], [1,6], [7,12]]. One arrow at $6$ bursts [2,8] and [1,6]. One arrow at $11$ bursts [10,16] and [7,12]. Total: $2$ arrows.

How is this related to activity selection? Tricky: minimum arrows = number of non-overlapping groups. Each group of overlapping balloons needs one arrow.