如图所示，MN是流速稳定的河流，河宽一定，小船在静水中的速度大小一定，现小船自A点渡河，第一次船头沿AB方向与河岸上游夹角为α，到达对岸；第二次船头沿AC方向与河岸下游夹角为β，到达对岸，若两次航行的时间相等，则（   ）

    Sam's parents took her to the World Park last week. He saw Big Ben, the Tower Bridge, Disneyland in America, the CN Tower, Niagara Falls and the Sydney Opera House. His father told her Big Ben and the Tower Bridge are in London. Her mother told her Niagara Falls is in Canada and the Sydney Opera House is in Australia. He learnt about many things of the famous places around the world.

小明在探究“小灯泡发光时的亮度与电功率的关系”的实验中，所用的电源电压是4V，小灯泡的额定电压是2.5V，小灯泡两端允许加的最大电压为额定电压的1.2倍。