A:∃x1,x2∈R,(f(x2)﹣f(x1))(x2﹣x1)≤0
B:∀x1,x2∈R,(f(x2)﹣f(x1))(x2﹣x1)≤0
C:∃x1,x2∈R,(f(x2)﹣f(x1))(x2﹣x1)<0
D:∀x1,x2∈R,(f(x2)﹣f(x1))(x2﹣x1)<0
魏晋南北朝时期,我国人口移动的方向是( )