Geometry

Question Analysis:  
This question involves classifying a triangle based on angle measures.  
The main focus is recognizing an obtuse triangle, which has one angle greater than 90°.  

Key Concept Explanation:  
- Acute Triangle: All angles are less than 90°.  
- Right Triangle: One angle is exactly 90°.  
- Obtuse Triangle: One angle is greater than 90°.  
- Equilateral Triangle: All angles are 60°.  

Step-by-Step Solution:  
1. Identify the given angles: 100°, 40°, and 40°.  
2. Since one angle is greater than 90°, the triangle is an obtuse triangle.  
3. Checking the answer choices:  
  - (A) Acute Triangle: Incorrect, because an acute triangle has all angles < 90°.  
  - (B) Right Triangle: Incorrect, because there is no 90° angle.  
  - (C) Obtuse Triangle: Correct, because one angle is greater than 90°.  
  - (D) Equilateral Triangle...