Question

Which does not contain a run-on sentence?

a. The Big Dipper is almost always visible in the Northern Hemisphere, so these stars make a good reference point.
b. The Big Dipper is almost always visible in the Northern Hemisphere these stars make a good reference point.
c. The Big Dipper makes a good reference point, these stars are almost always visible in the Northern Hemisphere.
d. The Big Dipper is almost always visible, these stars make a good reference point in the Northern Hemisphere.

Answer 1

A. It changes keys smoothly from one side of the sentence to the next. ", so" is a coordinating conjuction. It's sort of like the point that connects two train cars to each other. This also keeps it from being a run on sentence.

(Other conjuctions are: and, but, for, nor, or, and yet.)