单项选择题
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
A．they look like fare inspectors B．they don’t wear uniform C．they are polite to passengers D．they often take a back seat