The answer is C because you can have a triangle with 5,9,n 5,10,n 9,10,n and 5,10,9
The reason why is because the sum of two sides is always larger than the value of the third side.
I hope that this helps.
Complement means the other angle that will sum up to 90°
So:
x + 2x = 90°
3x = 90
x = 30°
So, it is 30° and 30 * 2°, or 60°
