Read the following paragraph carefully and choose the best topic sentence from the four possible answers that follow the paragraph by writing A, B, C or D.
Since there are more than 80 million current and former smokers in the United States, a widespread CT scanning program of at-risk individuals would De enormously costly. Low-dose CT scans also expose patients to considerably more radiation than standard chest X-rays do, and little is known about how the cumulative risks of years of such scans would balance out with the benefits of the information they provide. Also, more scans would also mean more false positive results, leading to more additional tests and surgical procedures performed on people who do not turn out to have any disease, further complicating the picture.
A、CT scans should be improved in more than one way.
B、CT scans can be very costly in the United States.
C、CT scans may not present clear picture for patients and doctors.
D、There are disadvantages in using CT scans to fight diseases.
Topic sentence : _________________________