$(a)$ What is meant by fossil fuel?
$(b)$ Which fossil fuel is usually conserved when we save on electricity?
$(c)$ At the present rate of consumption,how long will the known reserves of coal and petroleum last?

(N/A) Fossil fuels are non-renewable energy resources formed from the degradation of plant and animal biomass buried deep under the Earth millions of years ago.
$(b)$ Coal is the fossil fuel usually conserved when we save electricity,as most power plants in many regions rely on coal combustion for energy generation.
$(c)$ At the present rate of consumption,the known reserves of coal are estimated to last for approximately $200$ years,while the reserves of petroleum are estimated to last for about $40$ years.