[单选题]
A mass is attached to a vert.f ndzvvd+u-j(2 2inq+q:owkical spring and bobs up and down between points 0 jncif9ekt12A and B. Where iseckt j29ni0f1 the mass located when its potential energy is a minimum?
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