The lengths of AB and BD can be determined using the number line. AB can be determined by the distance between points A and B, which is 4 units. BD can be determined by the distance between points B and D, which is 5 units.
According to the Triangle Inequality Theorem, for any triangle, the sum of the lengths of any two sides must be greater than the length of the third side.
From the options given, the two segments that could be used as the third side of the triangle are CD and AC. Therefore, the correct response is "CD and AC."