Choose the Most Correct Statement Concerning Co-60 Teletherapy $\gamma$ -Rays Whose Average Energy Is 1

Choose the most correct statement concerning Co-60 teletherapy.

A) A timer error factor must be added to the treatment time.
B) The head contains the source,shielding,and collimators.
C) The Co-60 source is 1 to 3 cm in diameter.
D) Co-60 emits two $\gamma$ -rays whose average energy is 1.25 MeV.
E) All of the above are true.