单项选择题 You are underway in thick fog. You have not determined if risk of collision exists. Which statement is TRUE?（）.

单项选择题 You are underway in restricted visibility and hear a fog signal forward of the beam. Nothing appears on your radar screen. You must（）.

单项选择题 You are underway at 5 knots and see on your radar a contact 10 miles directly astern of you. 12 minutes later，the contact is 8 miles directly astern of you. What is the estimated speed of the contact? （）