Which of the following is a sentence fragment?
a. knowing he would be late, she took a nap.
b. realizing you are getting older is sobering.
c. hiding in the tree fort he had built as a kid.
d. checking his mail each day was always fun.

fire

explanation:

the direction of the magnetic field produced by an electric current will change when the direction of the electric current changes. for example : if a current is flowing in a straight length of wire, the magnetic field is described by a concentric circle (using right-hand rule).