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.