Which rock layer contains the oldest fossils? 1. a2. b3. c 4. d

4. D since its basically the bottom one

Yeah it would be 4. D
The other ones are younger because they are higher up in the ground.

The surface of the early Earth is often described as hellish by scientists. The high temperature is primarily attributed to the high levels of carbon dioxide in the atmosphere and the molten rock on the surface of the planet. However, these two aren't the only factors in play.  
Another factor that contributed to the extremely high temperature of the early earth is radioactivity. Radioactive decay of the isotopes found underground plus the left over heat from the formation of our planet produced high levels of radiogenic heat.
There's also the kinetic energy from falling asteroids and meteorites. Massive amounts of energy in the form of heat and sound is produced when asteroids and meteorites hit the Earth. Also, the pressure waves that travel radially outwards the center of the impact is similar to the impact of an atom bomb. As such, dust particles envelop the atmosphere, trapping the heat from the impact. 
The last factor is the gravitational contraction, it contributed to the hellish temperature of primordial Earth because the energy generated from such event is enough to trigger hydrogen fusion.