[单选题]
A mass is attached to a vertical spring and bok681o h kbpl;mbs up ak )h.2owbd6 gund down between points A and B. Where is the massgw k 2ob)6.hdu 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