thE nAvigAtion ComputEr FunCtions ______.
A．By mEAns oF A DirECtion FinDEr AnD A spEED DEtECtor
B．BAsiCAlly on sAtEllitE signAls AnD A mAp DAtABAsE
C．mAinly through thE rECEption oF turn-By-turn DirECtions
D．By using A sCrEEn to DisplAy sAtEllitE signAls