The correct statements are:
3. Dinosaurs evolved after Precambrian organisms;
5. Precambrian organisms were the first life-forms to become fossilized, according to this sample;
The Precambrian organisms were the ones that have left the first fossilized evidence of their existence, thus being considered as the first organisms that appeared on Earth.
In the periods after the Precambrian, lot of other life-forms developed, including the dinosaurs and the mammals which appeared in roughly the same period. The dinosaurs fossils being dominated though as they managed to become the most numerous and dominant animals on the planet, while the mammals were very small and living in their shadows.
After the dinosaurs went extinct, it was the mammals's turn to become the dominant life-forms, so in the top layer A, they are the most dominant fossils.
