21. 根据实物测绘机床电气设备电气控制原理图时，同一电器的各元件(  )。
A、 A、要画在1处  
B、B、要画在2处
C、 C、要画在3处  
D、   D、根据需要画在多处

A．In block organization, you can write about similarities and differences by subtopic.
B．The block pattern is a good approach for long essays.
C．For shorter essays, point-by-point organization may be more appropriate.
D．The point-by-point pattern is essential if your material is complicated.