Which of the following sentence is right?
A.
The goal is to learn their risk of thyroid cancer the extent to which radiation may increase .
B.
The goal is to learn the extent to which radiation may increase their risk of thyroid cancer.
C.
The goal may increase their risk of thyroid cancer is to learn the extent to which radiation .
D.
The goal is to learn the extent to radiation which may increase their risk of thyroid cancer.