有一重为200N的正方体物体放在水平地面上，它的边长为20cm，小明想通过如图所示的定滑轮来竖直提起这个物体。（g取10N/kg）求：　　

（1）该物体的密度是多少？

（2）当绳子中的拉力为零时，物体对水平地面的压强是多大？

（3）当人用250N的力向下拉绳子时，物体恰好能匀速上升，物体上升3m后恰好到达目标位置。此过程中人所做的功和定滑轮的机械效率是多少？

(1)analysis emphasizes the drawing of pictorial system models to document and validate both existing and/or proposed systems. Ultimately, the system models become the (2)for designing and constructing an improved system. (3)  is such a technique. The emphasis in this technique is process-centered. Systems analysts draw a series of process models called (4).(5) is another such technique that integrates data and process concerns into constructs called objects.

空白（1）处应选择（）

A.Prototyping

B.Accelerated

C.Model-driven

D.Iterative