99色精品-9色在线-99色在线-99色视频 国产欧美日产一区二区三区_亚洲精品亚洲人成在线观看_四虎在线精品永久观看_免费一级a一片久久精

單項(xiàng)選擇題

The period of a lighted aid to navigation refers to the().

A.date of construction or establishment
B.length of time between flashes of the light
C.time required for the longest flash of each cycle
D.time required for the light to complete each cycle

微信掃碼免費(fèi)搜題