Trapezoid A B C D can be divided into three equilateral triangles.
If the perimeter of the trapezoid is equal to 840 \mathrm{~cm}, what is the length of A B ?
Answer Choices
A. 120 \mathrm{~cm}
B. 140 \mathrm{~cm}
C. 168 \mathrm{~cm}
D. 25 \mathrm{~cm}
E. 210 \mathrm{~cm}