My 2 cents:
C) With the exception of elite runners, times for whom are calculated from the starter's pistol to the finish line,
D) Unless the runner is elite, in which case the time is calculated from the starter's pistol to the finish line,
...marathon times are calculated using a computer chip attached to each competitor's shoe.
The i choose D instead of C was that the structure of D was somewhat sensible -sub-ordinate clause, modifier, main clause
In C we have - modifier, non-essential part,
...
C) With the exception of elite runners, times for whom are calculated from the starter's pistol to the finish line,
D) Unless the runner is elite, in which case the time is calculated from the starter's pistol to the finish line,
...marathon times are calculated using a computer chip attached to each competitor's shoe.
The i choose D instead of C was that the structure of D was somewhat sensible -sub-ordinate clause, modifier, main clause
In C we have - modifier, non-essential part,
...
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