Sales OHaganBooks.com modeled its weekly sales over a period of time with...

Sales OHaganBooks.com modeled its weekly sales over a period of time with the function

as shown in the following graph (t is measured in weeks):

a. As time goes on, it appears that weekly sales are leveling off. At what value are they leveling off?

b. When did weekly sales rise above 10,000?

c. When, to the nearest week, were sales rising most rapidly?