10. what is commonly used to run a central heating system?
geothermal energy
solar energy
electrical energy
nuclear energy

Electrical energy is commonly used to run a central heating system.

Explanation:

Central heating is a method of heating a building, where heat is produced centrally and then distributed to different sites through a heat distribution system.  

A typical construction is water heating. In it, an electric water-heating boiler reserves a hot water heater. The circulating pump or gravity recirculates the warming water in the accumulator in a closed water pipeline, which circulates the water in the radiators and releases heat to the various spaces in the desired amount.

answer; (d) spontaneous processes are characterized by the overall conversion of order to disorder;

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.