Two in a corner, 1 in a room, 0 in a house, but 1 in a shelter. What am I?

Answer: R

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More What am I ?

Two in a whole and four in a pair. Six in a trio you see. Eight’s a quartet but what you must get is the name that fits just one of me. What am I?
Two is one, four is two, and six is three. Don't you see? What am I?
Two legs I have, and this will confound, only at rest do they touch the ground. What am I?
U always follow me but I am rarely seen. What am I?
Under pressure is the only way I work, and by myself is the only way I'm hurt. What am I?
Until I am measured, I am not known. Yet how you miss me when I have flown! What am I?
Use me if you really want to see inside someone. What am I?
Use me to reward good behavior. What am I?
Violet, indigo, blue and green, yellow, orange and red; these are the colors you have seen after the storm has fled. What am I?
Water on wind; Light, I rescind. What am I?
We applaud if you perform well. What am I?
We hurt without moving. And poison without touching. We bear truth and lies, But are not judged by size. What are we?