Which of the following market structure characterized that a large number of competitors produce (slightly) differentiated products with low barriers to entry（）

A. Perfect competition.

B. Monopolistic competition.

C. Oligopoly.

当转塔型切边剪一侧在工作时，可在（）更换剪刃。

A.任意一侧

B.有刀具的一侧

C.空闲的一侧

D.刀箱的一侧