The expenses of a dormitory are partly fixed and partly variable

[Evo23]

The expenses of a dormitory are partly fixed and partly variable. It costs $12 per head per day if there are 20 students and $11 per head per day if there are 25 students. What will be the cost per head per day if the number of students increases to 40?

A. $8
B. $8.5
C. $9
D. $9.5
E. $10