The answer to this is B. Some people do not even know what Roentgen is. This is also known as rontgen. This is a type of measurement that is associated with gamma rays. This is something that cannot be avoided when people need to get Xrays but there are some who may try to avoid their exposure to it because of their belief that it can cause cancer.
It would take an exposure of about 1000 sievert in order to cause cancer. For example, someone who has gone through a CT scan more often for a short period of time may be at a greater risk of acquiring cancer as compared to someone who has not gone through a CT scan at all.