[单选题]
A mass is attached to a vertical spri w, 1zzphjb d/e+n8qvmv5p(s6ng and bobs up k.l o+- iv.hko/o-cyi and down between points A and B. Where is the mass .. hvc lkioky/+io- o-located when its kinetic 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