Explain why fossil fuels such as crude oils contain varying amounts of nitrogen and sulfur.

Fossil fuels have resulted from the anaerobic decomposition of the organisms that died millions of years ago. The dead organisms (including plants and animals) were covered by layers of sediments and thus underwent decomposition in the absence of oxygen. Since these fuels have resulted from dead organisms, their constitution results from the elements present in these organisms. Some of the commonly used fossil fuels are coal, crude oil, and natural gas.
The crude oil contains 0.05%-0.5% sulfur and 0.5%-2.1% nitrogen. In fact, sulfur is the third most abundant element in the crude oils and can have concentrations of up to 14% in heavier crude oils.
It is the presence of these elements (sulfur and nitrogen) that causes the air pollution when crude oil or its resultant products (such as, diesel, petrol/gasoline, etc.) undergo combustion (typically in vehicles and/or industrial applications).
Hope this helps.