An epidemiologic survey of roller-skating injuries in Metroville, a city with a population of 100,000 (during the midpoint of the year), produced the following data for a particular year:

Number of skaters in Metroville during any given month 12,000
Roller-skating injuries in Metroville 600
Total number of residents injured from roller-skating 1,800
Total number of deaths from roller-skating 90
Total number of deaths from all causes 900

The proportional mortality ratio (%) due to roller-skating was:

a. 90/600 × 100
b. 90/100,000 × 100
c. 90/1,800 × 100
d. 90/900 × 100