There are 3 ways the department will handle the situation.
1 Do nothing, which D would be at 40%, C at 50%
2 Move the scale down, lets say drop D down to 30% and C to 40%
3 Use the Geometric Mean, which depends on the performance of the students.
Lets say most people get 30% of the total score, which D and C would be around 25% - 35%
Right now the department is doing nothing and waiting for the final result from students to see which option they will choose.
If most of us do badly in the final, option 2 or 3 will be most likely.
That being said, I hope we all do badly.