Geometry

Question Analysis：
This is a concept-based classification question.
The main focus is to test your knowledge of defining properties of parallelograms.

Key Concept Explanation：
In a parallelogram:
Opposite sides are always parallel
Other properties (like right angles or diagonals being perpendicular) only apply to special cases like rectangles or rhombi.

Step-by-Step Solution：
Examine each option:
A: Right angles? Only in rectangles
B: All sides equal? Only in rhombi
C: Perpendicular diagonals? Only in rhombi or squares
D: O...