Several photographers named Christensen were found, four in Iowa, Minnesota, and Nebraska. This item from the Bulletin of Photography vol 22 - 1918 makes A C Christensen seem the best choice: “A. C. Christensen of Nelson, NE has disposed of his studio to H F Galusha.”
Nelson, NE, is only about 80 miles from St. Paul, NE, but the only A C Christensen found, although he lived in St Paul, NE, 1900-1910, was not born until 1891, much too late to be the photographer here.
So at this point, the Christensen credited here was probably an itinerant photographer based in another larger town. St Paul, NE, was on a rail line from Grand Island, NE, and other points, such as Duluth, MN.
Evidence so far indicates the photographer Christensen of Nebraska was the same as the Christensen of Minnesota.
25 West Superior Street, Duluth, MN
example from Cabinet Card Gallery