1. (10 points) Two small triangles placed without overlapping can form a larger triangle, then this larger triangle cannot be formed by ( ).
A. Two acute triangles
B. Two right triangles
C. Two obtuse triangles
D. One acute triangle and one obtuse triangle
【Analysis】Since a straight angle is $180^{\circ}$, two small triangles that fit together must have two sides collinear, which can form a straight angle at this time, so the sum of the two angles must equal a straight angle, and the answer can be obtained accordingly.

【Solution】Solution: Since two small triangles that fit together must have two sides collinear, at this time a straight angle can be formed,
A, Since the sum of two acute angles is less than 180 degrees, therefore, two acute triangles cannot form a large triangle;
B, Since $90^{\circ}+90^{\circ}=180^{\circ}$, two right triangles can form a large triangle;
C, Since obtuse angle + acute angle may equal $180^{\circ}$, two obtuse triangles may form a large triangle;
D, Since obtuse angle + acute angle may equal $180^{\circ}$, two obtuse triangles may form a large triangle; therefore, the answer is: $A$.