Which type of central heating system is often used when heating many buildings from a central location?
a. electric baseboard
b. forced-air
c. hot-water
d. steam

Steam is often used as a central heating system when heating many buildings froma central location. So the correct option for the given question is option "D". Steam is actually generated ina single room or place mechanically or by useing fossil fuel or solar energy and then it is passed through pipelines to several rooms and buidings. This kind of system is mostly used when heating several rooms or building at once. Electric heaters are normally avoided as the cost of electricity is far higher than any other source of heating.

Protons have a positive charge and, collectively, give an element its atomic number.