# Dense

Suppose that B is not dense in A. Then there is a point $x\in A$ and a ball U around x which meets no point of B. Since B is contained in A-U, $m(B)\le m(A-U)$. Now $m(A) = m(B) = m(A-U) + m(U) \ge m(B) + m(U)$, but m(U) must be positive, a contradiction.