[单选题]
A mass is attached to a vertical spring and bobs .,xuc2 k/)eorrietd; up and down between points A yw )d xa*1rc0bwkz 5i/lc v.p7and B. Where is czwpk r7vi 1lcxya/0.*5)b wdthe 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