The graph below shows the mortality rate for a certain disease in the years since a new drug was introduced to combat it. If t is the number of years since the drug was introduced and m is the mortality rate for the disease, which of these equations does the graph represent? The graph shows 'Years since drug was introduced', from 0 to 8 along the x-axis and 'Mortality rate for the disease', from 0 to 1 in increments of 0.1 along the y axis has a downward sloping curve passing through the points (1, 0.4), and (3, 0.1). A m=0.5t+0.8 B m=0.5(0.8t) C m=0.8t+0.5 D m=0.8(0.5t)