Here's why:

* Active margins are zones where tectonic plates are interacting, resulting in geological activity like earthquakes, volcanoes, and mountain building.

* Rift valleys are formed by the divergence of tectonic plates. This divergence causes the Earth's crust to stretch and thin, leading to the formation of a valley.

* This divergence often leads to volcanic activity as magma rises from the mantle to fill the space created by the separation.

* Earthquakes are also common in rift valleys, due to the movement of the plates and the adjustments of the crust.

Therefore, the presence of active geological processes like volcanism and earthquakes makes rift valleys a clear example of active margins.