单项选择题
A．keep a close watch over the road ahead B．take pictures of the passengers on board C．increase the efficiency of the transit system D．help the transit police detect criminals
A．it is still possible that your tour may be cancelled B．you are still allowed to change your visiting date C．you can surely visit the White House that day D．it is necessary for you to start your tour immediately
A．exactly on the date of application B．at least 30 clays in advance C．any day between Tuesday and Saturday D．10 days before the date of the tour