7) The answer is $(\mathrm{B})$.

In any triangle $A B C$, the length of $A B$ is greater than or equal to the length of $B C$ minus the length of $A C$ (assuming $B C \geq A C$), and it is equal to this difference only if $A B C$ are collinear and $A$ is between $B$ and $C$.
7) The city of Mystery is $500 \mathrm{~km}$ from Mouseville and $1200 \mathrm{~km}$ from Duckburg. What is the minimum possible value for the distance between Mouseville and Duckburg?
(A) $500 \mathrm{~km}$
(B) $700 \mathrm{~km}$
(C) $1200 \mathrm{~km}$
(D) $1300 \mathrm{~km}$
(E) $1700 \mathrm{~km}$.