Find the Avrage value of the function i=15(1-e to the power of -1/2 t) from t=0 and t=4?

A.7.5 sqrt 1+4e^-2 -e^-4 <<<<not sure though
B.7.5 sqrt 4-2e^-2 + e^-4
C.7.5 sqrt 4+4e^-2 - e^-4
D.7.5 sqrt 5-2e^-2 + e^-4

I integrated the expression

15(1-e^(-t/2))
and got: 15(1-e^(-t/2)).

When evaluated from 0 to 4 I got:
30*%e^(-2)+30
Divide by (4-0) gives 7.5(e^-2+1)=8.5

Could there be a typo somewhere?