Answer:
A. 5, 7, 9
Explanation:
in a regular triangle the sum of any 2 sides must always be greater than the third side.
A.
5+7 = 12 > 9
5+9 = 14 > 7
9+7 = 16 > 5
yes, this can be a triangle.
B.
1+8 = 9 = 9
that violates the condition. both sides together are equally long as the third side, so the triangle would be only a flat line with the top vertex being squeezed flat onto the baseline.
no triangle.
C.
5+5 = 10 < 12
that violates the condition. the sides cannot even connect all around.
no triangle.