Although High Doses of Radiation Are Harmful to Cells,how Can

Although high doses of radiation are harmful to cells,how can low levels of radioactive isotopes be used medicinally?

A) as tracers in imaging organs using Xrays and PET Scans
B) to reduce obesity and diabetes in teenagers
C) to create drugs that work faster than normal
D) to destroy aging and unwanted cells
E) to prevent ultraviolet damage from the sun