A. flashing red lights upstream and fixed red lights downstream B. yellow unlighted buoys C. signs and/or flashing red lights D. red daymarks upstream and green daymarks downstream
A. 2 B. 3 C. 4 D. 5
A. 教学的物理环境 B. 教学的心理环境 C. 教学的社会环境 D. 教学的文化环境