[单选题]
A mass is attached to a9ni qczn*f :+d vertical spring anosdb ub ej sn16;bm-4x8qg*8yd bobs up and down between points A and B. Where i8 o*sgsbbxe qb; dm8n1-ju6y 4s the mass located when its potential energy is a maximum?
A. at either A or B
B. midway between A and B
C. one-fourth of the way between A and B
D. none of the above
