执行机构的开关方向标识清晰，拉杆动作灵活、平稳、无卡涩，拉杆长度应大于5m。
参考答案：错
The great invention he made _____ of great benefit to the whole world.[ ]
A. is proved
B. has proved
C. has been proved
D. is proved to be
并发
