Which front brings several days of steady rain or snow into an area? A. a cold front
B. a warm front
C. a stationary front

i guess that visual aids present persuasive examples to what you are presenting, some factors would be theme (does it fit with the color, information, what you are presenting), size (can you see it, is it a big enough span of data/information), and overall look of the visual aid.

hope this : )

inverse v=i*r

nice nice