Directions: Choose the best item to complete the missing part.
Passage 2
Fossils are a faithful recording of __14__, an accurate imprint of the past. __15__, imagine a prehistoric fish swimming around in the sea a few million years ago. One day, the fish dies __16__ its body sinks down to the water bottom, where it becomes buried in soft mud.
__17__ time passes, the flesh decays and __18__. An imprint of the skeleton stays in the mud, even after the skeleton itself disintegrates. Many years pass. The mud with __19__ gradually hardens and turns to stone under the pressure of the rock layers that have subsequently built up on __20__.