Gary balanced his pencil on the back of his chair. Which of the following is true?
A.
The pencil must fall off the chair eventually, due to gravity.

B.
If the pencil falls off the chair, this means an outside force was applied.

C.
The pencil must be magnetically attracted to the chair.

D.
If no outside force is applied to the pencil or chair, then the pencil will fall down.
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