“五一”小长假，中央电视台千岛湖水下古城探秘大型直播把很多人吸引在电视机前．某科学兴趣小组通过查询资料进一步了解有关该事件的信息，这次探秘的主要任务是由机器人下水完成，该机器人的有关数据：体积为0.2立方米；工作水深300米；水下最高速度3节（1节=1.852km/h）．（g=10牛/千克）

亚洲和非洲的分界线是巴拿马运河。

    A little knowledge (知识) can make a difference at a time of life or death. Do you know what to do if you find yourself in an earthquake?
If you are inside:
●Quickly move under a strong desk, a strong table or along a wall. You can also go into a small room like the bathroom. Do not get into a lift.
●Don’t stand near windows, large mirrors, heavy furniture or fireplaces.
If you are outside:
●Move away from buildings and streetlights. These could fall on you.
●Squat (蹲下) and protect your head until the shaking stops.
After the earthquake:
●Check, take care of injuries (损伤) and help make sure people around you are all right.
●Watch out for aftershocks.
●Protect yourself by wearing long pants, a long-sleeved(长袖的) shirt and strong shoes.
●Listen to the radio for the latest information.

形如$\left|\begin{array}{cc}a& c\\ b& d\end{array}\right|$式子叫作二阶行列式，它的运算法则用公式表示为$\left|\begin{array}{cc}a& c\\ b& d\end{array}\right|$=ad﹣bc，依此法则计算$\left|\begin{array}{cc}2& 1\\ -3& -4\end{array}\right|$的结果为（　　）