Human Skin Is Very Sensitive to Radiation

Human skin is very sensitive to radiation. Which of these precautions is taken to prevent the skin from being severely affected while being administered a radiation beam from a source to a tumour?

A) In order to avoid skin damage, the radiation source is injected under the skin.
B) Even though the radiation source is at a distance from the skin, the radiation reaches maximum dose under the skin because of an initial build-up region, which causes lower doses on the skin.
C) The beam is focused so finely with collimators that the skin is damaged, but only in a very narrow area.
D) The source of radiation is placed on the surface of the skin and, because of the initial build-up region, the dose on the skin is lowest.