What is the difference in the beginning between primary succession and secondary succession

Which element is used in photosynthesis found in atmosphere and fixed by bacteria?

Fossils can only be formed when? if mud, silt or tar is in the area to preserve the fossil ierosion does not occur and allows the fossil to form oxygen and microorganisms cannot decay the bones the area is dry and does not contain mud or silt

Each pair of clay balls represents two planetesimals. if each planetesimal is composed of the same material and is separated he the same distance, which pair experiences the frayed gravitational attraction?