A force field in the xy-plane is given by F=6xi^+4yj^N. Which one of the following potential energy functions U(x,y) (in joule) correctly corresponds to this force? (Assume the usual relation F=−∇U.)
a) U=3x2+2y2+C
b) U=−3x2−4y2+C
c) U=−3x2−2y2+C
d) U=−2x2−3y2+C