单项选择题
The unit of duration of a dash in Morse Code is（）.
A.one and one-half times the length of a dot B.twice the length of a dot C.three times the length of a dot D.four times the length of a dot
单项选择题 The range of a SSB transmission is MOST affected by（）.
单项选择题 The light rhythm of Morse (A) is shown on（）.
单项选择题 The following（）is a standard phrase.
