这种黄黑相间的倾斜线条是什么标记？（）
A．减速标记
B．实体标记
C．突起标记
D．立面标记
参考答案：D
Han Han's books are popular. They __________ by many teenagers.
A. is read
B. is invites
C. were read
D were invites
铸铁中强烈阻碍石墨化的元素是（）。
A、碳
B、硅
C、锰
