题目

Mr. Brown first went to look at the underground-fire when he was seven. “ Through the hole in the earth, you could see the orange fire, but you had to look fast because it was so hot,” said Mr. brown. In 1898, he saw the fire once more. Now, we can only see the smoke. The fire Mr. Brown saw is not the underground-fire. In fact, there are 260 coal fires in the world. They are harmful and dangerous. Then how to put them out?     Scientists have tried to set fire to underground coal to speed up the fires. In this way, the burning of underground coal would be soon finished. 45. Mr. B如图甲所示为一列简谐横波在t=20s时的波形图，图乙是这列波中P点的振动图线，那么该波的传播速度和传播方向是A．υ=25 cm/s，向左传播        B．υ=50 cm/s，向左传播C．υ=25 cm/s，向右传播        D．υ=50 cm/s，向右传播