[单选题]
Liquid flows through a 4.0 cm diameter pipe at 1.0 mx9fv3p(d 76jt iq z.z. fhgw6i/s. There is a 2.0 cw u9e0e4n29b lzld;gu:l cmdns*1 r4y m diameter restriction rdzueb 2*90 4ues1l4n9dnlly ; wmc:gin the line. What is the velocity in this restriction?
