At a certain section in a pipeline, a reducer is used to reduce the diameter from 2D gradually to diameter D. When an incompressible fluid flows through this pipeline, the velocity is U1 in the first section and U2 in the second section. Which of the following is a true conclusion?
a. U2 = 4U1
b. U2 = 2U1
c. U2 = U1/2
d. U2 = U1/4