Bacteria and other prokaryotes are found deep underground in water and in the air. one micron 10^-6 is a typical length scale associated with these microbes

I don't think there is enough information from the question. If you are asked to calculate the number of microbes in a certain area., then it should be that the total area or maybe the volume of that should be given so you can estimate the number of microbes. Hope this helps.

The answer is D. They are more conductive than nonmetals but less conductive than metals. 

A semiconductor is a solid substance with a relative conductivity that is higher than insulators but considerably lower than conductors. Metals are known for their good conductivity and non-metals are known for their poor conductivity. This is why the metalloids are semiconductors. Metalloids are elements that have some properties of metals and metalloids.