AMC 10 2008b Problem 7

An equilateral triangle of side length $10$ is completely filled in by non-overlapping equilateral triangles of side length $1$. How many small triangles are required? $\mathrm{(A)}\ 10 \mathrm{(B)}\ 25 \mathrm{(C)}\ 100 \mathrm{(D)}\ 250 \mathrm{(E)}\ 1000$