A certain book had an original rating of 27 out of 30 points on a survey chart. Each month, the rating fell by 3 points. Express the number of points as a function of m, the number of months since the release.

(A) f(m)=3m (B) f(m)=3m−27 (C) f(m)=30−3m (D) f(m)=27−3m (E) f(m)=3−27m