Fact 1: Adult sponges are sessile.

Fact 2: Hydras are sessile, attaching to some sort of substrate.

Given the two facts above, answer the question "Where do adult sponges attach themselves?" with the following options: 
- animal
 - stalk
 - Sediment
 - coastlines
 - plant
 - substrate
 - ocean water
 - loose soil


 

      substrate   