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:

Mold and cast fossils form when an organism is buried in sediment. In still - water environments, sediment can gradually build up over organisms. As the organism decays (usually the soft tissue decays first), it leaves an impression (mold) in the sediment. Then, if minerals fill that mold, a cast is formed. The key is that sediment needs to accumulate over the organism. Option 2 is incorrect as sediment does not turn organisms into rock in the context of mold and cast fossil formation. Option 3 is wrong because hard tissue is more likely to be part of the fossil record (if not decayed) but the process of mold and cast formation is about the sediment build - up. Option 4 is incorrect as soft tissue is not preserved in mold and cast fossils (it decays).

Answer:

Sediment builds up over organisms.