******在网络安全和信息化工作座谈会上指出,维护网络安全,首先要知道风险在哪里,是什么样的风险,什么时候发生风险,正所谓“聪者听于无声,明者见于无形”。下列与这句引用语所蕴含哲理一致的是( )
①居安思危,思则有备,有备无患
②不积跬步,无以至千里;不积小流,无以成江河
③宜未雨而绸缪,毋临渴而掘井
④旧时王谢堂前燕,飞入寻常百姓家
A:①②
B:①③
C:②③
D:③④
The farmer is very busy because he's so _______ sheep to keep and so ______ work to do every day.
下列叙述中,正确的是 ( )
Finding information in today's world is easy. The is how you can tell if the information you get is useful or not.
一轻质杠杆可绕O点转动,在杠杆的A点和B端分别作用两个力F1、F2,已知OA:AB=1:2.
若F1=12N,方向竖直向下,为使杠杆保持水平平衡,求作用在B端的力F2的方向和最小值.