— Could you teach me _______French?
— Of course. But we must choose a right time for _______.
A:learn, it
B:learn, one
C:to learn; one
D:to learn; it
圆的周长与半径的关系为:C=2πr,其中自变量是____
①秦朝 ②隋朝 ③唐朝 ④元朝