D. 90 is the answer.
What is congruency in maths?
Two geometric figures are congruent or related to congruence if one of them can be superposed to match exactly.
In geometry, if two shapes or objects have the same shape and size, or if one has the same shape and size as the mirror image of the other, they are congruent.
The word "joint" means "exactly the same shape and size". If you rotate, flip, or flip the shape, it stays the same. For example, draw two circles with the same radius, cut them out, and place them on top of each other.
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