To determine the appropriate unit for the average speed of a runner during a half-marathon, we need to consider the common units used in running events. A half-marathon is 13.1 miles long.
The average speed would typically be expressed in miles per hour (mph). Given that the distance of the race is measured in miles, the correct choice to fill in the blank would be:
B. miles
So, the complete sentence would read: "The average speed of the runner was 1.9 x 10^-3 miles."