If a reader had the following questions which strategy should he or she use to monitor understanding

either a donut or a sphere or a pancake or a flat sphere. it depends on where the vertical line is.

if the line goes through the center of the circle and is perpendicular to a diameter containing the center, then you get a flat circle.

if the line and the diameter of a the circle meet in the center of the diameter and at an angle, you get a pancake.

if the line is on the circumference of the circle and is at right angles   to the circle you get a ring.

if the line contains the diameter, you get a sphere.

the the line is parallel to the diameter and touches the circumference, you get a donut.

o'henry used irony in "the gift of the magi". this tool was essential to developing the story's theme by showing that the couple bought gifts for each other that they could not actually use at the moment. in addition, the author used this fact to show that people of the society should treasure the things they've received from others, because you may never know if they sacrificed their most treasured possession to aquire the gift.