[单选题]
If the position versus time gra 9k8x0yph nhel4ep(-ns5d k9iph of an obmkxfni /(2;nze1fbq-u f65 g hject is a vertical line, the object isqu;i2gk fmnb/ne(6fx-5 1 hzf
A. moving with constant non-zero speed.
B. moving with constant non-zero acceleration.
C. at rest.
D. moving with infinite speed.
