Hi, need help for this MCQ Question

Given:

100,000 men were encouraged to perform annual screening for cancer: 40,000 accepted, but 60,000 refused.

After 5 years, death rates from cancer (per 10,000) were 1.1 for the men who were screened, and 2.5 for the men who refused.

It is known that poorer men were less likely to accept screening than richer ones, and that cancer affects the rich more than the poor.

To show that screening reduces the risk from cancer, someone wants to compare the death rates from cancer among the screening group and the non -screening group,

which are 1.1 and 2.5.

Only based on the given information, 2.5 - 1.1 = 1.4 is likely ______ of the reduction in death rate from breast cancer due to screening.

a. underestimate why ?

b. reasonable estimation why ?

c. overestimate why ?