epidemiologic survey

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 cause-specific mortality rate from roller-skating was:

90/600 × 100,000

90/100,000 × 100,000

90/1,800 × 100,000

90/900 × 100,000