A converging lens is also known as a

•convex
•concave
•double
•refracted

Convex lens

Explanation:

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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.

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if the thermos has a vacuum innner, then heat through conv currents and conduction are reduced = vacuum flask