4. Two students standing on skateboards with frictionless bearings face the same direction
with student x behind student y, as shown above. If student x pushes on student y's back,
which of the following is true?
(A) Both students roll backward
(B) Student x rolls backward and student Y remains still
(C) Student X rolls backward and student Y rolls forward
(D) Student X remains still and student Y forward
(E) Neither student moves