答案:B
Which condition represents the appropriate time for setting off distress flares and rockets? ()
A.Only when there is a chance of their being seen by rescue vessels
B.At half hour intervals
C.At one hour intervals
D.Immediately upon abandoning the vessel
有关哈雷彗星的叙述正确的是()
①彗星是在扁长轨道上绕地球运行的一种质量很小的天体
②彗星呈云雾状的独特外貌
③哈雷彗星的公转周期是76年
④彗星绕日公转的方向与行星绕日公转的方向相同
A.①②
B.③④
C.②③
D.①④