A:1
B:1 18
C:1 12
D:12
-- Where is dad, mum?
-- He is ____his report in the room. He needs it for a meeting.
用数学归纳法证明“ 5n-2n 能被3整除”的第二步中,当 n=k+1 时,为了使用归纳假设,应将5k+1-2k+1 变形为____