In Paragraph 5, "two-tier system" means some ______.

A．men have many wives while others have none

B．people are very rich while others are very poor

C．people have too much work to do while others have nothing to do

D．people control others

使用加固线拉牵时，加固线与车底板夹角一般应成（）。

A.30°

B.45°

C.60°

D.90°