He goes to school the early bus.[ ]
A. on
B. can't be
C. must be
D. have to be
答案:A
一对啮合圆柱齿轮的节圆应(),在平行两轮的轴线方向投影为非圆的外形图中,啮合区节线应画为()
郑樵提出的类分图书的原则的哪四条?