Geometry

Question Analysis：
This question involves identifying the essential properties of parallelograms.
The main focus is on distinguishing what is always true from what is sometimes true.

Key Concept Explanation：
Parallelograms always have the following properties:
Opposite sides are parallel and congruent
Opposite angles are congruent
Diagonals bisect each other
However, not all diagonals are congruent or perpendicular unless the parallelogram is a special type.

Step-by-Step Solution：
A: ✅ Yes, by definition
B: ✅ Yes, proven by properties of transversals and parallel lines
C: ❌ No, only in rectangles/squares
D: ✅ Yes, always true
E: ❌ No, only true in rhombi/s...