Which of the following is true about mechanical practice in grammar practice? ( )

A、Mechanicalpracticeinvolvedactivitiesthatareaimedatformaccuracy.
B、Bydoingmechanicalpractice,thestudentspayrepeatedattentiontoakeyelementinastructure.
C、Substitutionandtransformationdrillsaremostfrequentlyusedinmechanicalpractice.
D、Thereisaclearcutbetweenmechanicalandmeaningfulpractice.