Two parallel rail tracks run north-south. Train $A$ moves north with a speed of $54 \; km \; h^{-1},$ and train $B$ moves south with a speed of $90 \; km \; h^{-1}.$ What is the velocity of $B$ with respect to $A$?

• A
  The train $B$ appears to $A$ to move with a speed of $10 \; m \; s^{-1}$ from north to south.
• B
  The train $B$ appears to $A$ to move with a speed of $40 \; m \; s^{-1}$ from south to north.
• C
  The train $B$ appears to $A$ to move with a speed of $10 \; m \; s^{-1}$ from south to north.
• D
  The train $B$ appears to $A$ to move with a speed of $40 \; m \; s^{-1}$ from north to south.