[单选题]
A mass is attached to a vertical spring and/9wo jj9dgdpb0)mzi ) mo z+uxwb1 327miaq1hbobs up and down between points A and B. Where is the mass located when i7x2u+zmoa bq h1 i3w1mts 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