How would you answer the following?

Below is a chart of the natural history of disease for one individual:

Years

>

ABCDEF

Key:

A = Biological onset

B = Disease detectable by screening

C = Individual's disease was detected by screening, the diagnosis was made, and treatment was begun.

D = Clinical symptoms would have developedand been diagnosed if the individual was not screened

E = Death would have occurred due to the disease if the individual had not been screened and treated

F = Death actually occurred

Which letters of the diagram are associated with the starting and stopping points for the lead time for this individual?

a.B to D

b.C to D

c.B to C

d.A to C