Question

Which sentence is a run-on sentence? A. Alaska is the largest state in the U.S., but it has the fewest people. B. Some foods are canned; others are frozen. C. Fencers wear metal helmets, these have wire face guards. D. Sarah swam quickly across the lake and back again.

Answer 1

I believe the answer is C. You can brake that sentence into to two sentences and it will flow better. 
Hope that helps! :) 

Answer 2

C
Fencers wear metal helmets, these have wire guards