Acitvity A has a finish-to-finish relationship with Activity B. Activity A has a duration of 7 days. Activity B has a duration of 5 days with a 3 days lag. what is the total duration of Activity A & B ?

A- 12 Days.

B- 8 Days.

C- 9 Days.

D- 7 Days.

The correct answer is B , Can anyone explain why it is B ?