It is the last choice because their similar bone structures suggest they once had a common ancestor, which is why they are all mammals.
If you were to compute this, Angle C should be equal to 100°.
In a triangle, all angles sum up to 180°.
In triangle ABC, where the m∠A =30° and m∠50°:
m∠A + m∠B + m∠C = 180°
30° + 50 + m∠C = 180°
80° + m∠C = 180°
m∠C = 180° - 80°
m∠C = 100°
The triangle should look like the picture attached.
The beginning of the Cambrian Period is marked by the evolution of hard body parts such as calcium carbonate shells. ... These body parts fossilize more easily than soft tissues, and thus the fossil record becomes much more complete after their appearance.