Question

why is still water an ideal environment for the formation of mold and cast fossils?
sediment builds up over organisms.
sediment turns organisms into rock
hard tissue decays under sediment
soft tissue is preserved under sediment

Explanation:

Sediment builds up over organisms. This sedimentation process is crucial for the formation of mold and cast fossils. As sediment accumulates, it can encase the remains of organisms. Over time, the sediment hardens, and the organic material may decay or be replaced, leaving behind a mold (an impression) or a cast (a filled - in impression). The other options are incorrect. Sediment does not directly turn organisms into rock (that's a more complex process involving mineral replacement beyond just sedimentation). Hard tissue does not decay under sediment in the context of fossil formation (it's more about the sediment - encasing process), and soft tissue is not typically preserved in the formation of mold and cast fossils (these fossils are more about the shape left by the organism, not the preservation of soft parts).

Answer:

Sediment builds up over organisms.