（单选题）
If your vessel is underway in fog and you hear one prolonged and three short blasts，this is a （）.
Avessel not under command
Bsailing vessel
Cvessel being towed (manned)
Dvessel being towed (unmanned)
正确答案
答案解析
略
Avessel not under command
Bsailing vessel
Cvessel being towed (manned)
Dvessel being towed (unmanned)
