cloud when it comes to reducing energy cost

Which one is the best for on-demand computing infrastructure when it comes to the saving cost?

a. The use of virtualization means that software runs faster and thus, uses less energy.
b. Locating computations in large data centers reduces the energy used by servers overall because they are physically closer together.
c. The use of virtualization means that software can be consolidate on few machines when usage is low, allowing us to shutdown unused machines, reducing energy use.
d. Economies of scale make cooling large numbers of machines cheaper.