单项选择题 Contour elevations on this chart refer to heights in feet above mean （）.

单项选择题 Navigational charts are subject to frequent changes，the important ones of which are promulgated by （）.

单项选择题 No navigational aids are shown and the chart is not kept corrected for alterations in depths inside the pecked lines. For more detailed information，the larger scale charts must be （）.