necessary クイズ

Skin needs ultraviolet radiation to begin the synthesis of vitamin D, but dermatologists say you can probably get the necessary electromagnetic input from a mere 20 minutes of sun exposure a week, as you go about your daily affairs, sunblocked and sans beach.

Now this most indispensable of all necessaries, after physical nutriment, cannot be had, unless the machinery for providing it is kept unintermittedly in active play.