单项选择题
Which statement concerning GPS is TRUE（）.
A.It cannot be used in all parts of the world B.There are 12 functioning GPS satellites at present C.It may be suspended without warning D.Two position lines are used to give a 2D fix
单项选择题 Which one of the followings does not limit the effective range of radar（）.
单项选择题 Which of the following statements is TRUE regarding crane operations（）.
单项选择题 Which of the following statements is TRUE regarding automatic identification systems（AIS）（）.
