The optical density of gross fog is:: (A) 0.6-3.0, (B) 0.2-0.3, (C) 0.2-0.6, (D) None of the above
Answer is B. An unexposed film, when processed, shows some density. This appearance is caused by the inherent density of the base and added tint and the development of a few unexposed silver halide crystals. This minimal density is called base plus fog and typically is 0.2 to 0.3. 
 
White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition