thE noisE-killing systEm is nEt yEt populAr mAinly BECAusE______.
A．it is still BEing triED out
B．DrivErs still hAvE thEir DouBts
C．it inCrEAsEs thE Cost oF CAr proDuCtion
D．CArmAkErs ArE not surE iF it is nECEssAry