已知电路的线图并选定树之后，在下列电流变量中，为一组独立变量的是( )

A、skateboarding
B、rock climbing
C、downhill mountain biking
D、acting

A、the students can turn their reports on the Monday
B、the reports on Monday could be received from the students by him
C、the students could hand in their reports on Monday
D、the students will on Monday the reports turn in