A. Having not discovered B. Not having discovered C. Having not been discovered D. Not having been discovered 参考答案:D 答案解析:分词作状语时,否定词一般要放在分词前。本句的主语是many laws of nature,它也是分词短语的逻辑主语,它们之间的关系是完成被动关系,因此选D项。
第21题: [单项选择]All fog signals shall be sounded every two minutes with the exception of a vessel(). A. underway or making way B. under sail or under tow C. anchored or aground D. not under command or restricted in her ability to maneuver 参考答案:C