Triangle A B C is an isosceles triangle with A B=B C. Point D is the midpoint of both \overline{B C} and \overline{A E}, and \overline{C E} is 11 units long. Triangle A B D is congruent to triangle E C D. What is the length of \overline{B D} ?
Answer Choices
A. 4
B. 4.5
C. 5
D. 5.5
E. 6