Points A, B, C, and D, in that order, lie on a line. If AB = 2 cm, AC = 4 cm, and BD = 6 cm, what is CD, in centimeters?
Choices:	A) 1	B) 2	C) 3	D) 4	E) 5
Putting a value to each point, lets use the following:   A - 0   B - 2 (AB = 2)   C - 4 (AC = 4)   D - 8 (BD = 6)   CD is 8-4= 
<gadget id="calculator">8-4</gadget>
<output>4</output>
4.   Ans 4
<result>D</result>
Points A, B, C, and D, in that order, lie on a line. If AB = 2 cm, AC = 4 cm, and BD = 6 cm, what is CD, in centimeters?