Which property is true for a rhombus but NOT for every rectangle?

Which property is true for a rhombus but NOT for every rectangle?
  1. Opposite sides are parallel
  2. Diagonals bisect each other
  3. All four sides are equal
  4. Opposite angles are equal

Solution

All four sides of a rhombus are equal. A rectangle has equal opposite sides but adjacent sides may differ.

Asked in: IMO

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