Which word does the underlined phrase modify? students found rules understand

Which does not contain a run-on sentence? a. using the big dipper, you can find the north star, just follow an imaginary line through the dipper's two end stars. b. using the big dipper, you can find the north star just follow an imaginary line through the dipper's two end stars. c. using the big dipper, you can find the north star by following an imaginary line through the dipper's two end stars. d. using the big dipper; find the north star just follow an imaginary line through the dipper's two end stars.