Which of the following statements is true about grammar practice?（ ）。

A、When doing transformation drills, the students also need to change the forms of the given prompts.
B、In substitution drills, the students change a given structure in a way so that they are exposed to another similar structure.
C、In meaningful practice the focus is on the production, comprehension or exchange of meaning.
D、An activity cannot have elements of both mechanical and meaningful practice.