①课上坐姿端正、专心听讲 ②在走廊、楼梯上要靠左行
③升国旗仪式时,要肃立致敬 ④只要下课铃一响,就可以马上离开教室去玩
A:①②
B:②③
C:①③
D:②④
I'm________about what I say because careless remarks are likely to hurt others' feelings.
如图是与生殖有关的结构示意图,请分析回答: