参考答案：C

解析：

[分析]： 数字信息的找寻和判断。 题干问绑架发生时有多少船员在船上。根据原文Unidentified kidnappers snatched two Norwegian and two Ukrainian oil workers 及there were 11 other crew members aboard the ship at the time of the kidnapping 将两句话中的人数相加，可知共有15名。[解题关键] 注意相关数字信息，并能够作简单的运算。