小河中有一个实心桥墩P，A为靠近桥墩浮在水面上的一片树叶，俯视如图所示，小河水面平静。现在S处以某一频率拍打水面，使形成的水波能带动树叶A振动起来，可以采用的方法是(　　)

A．提高拍打水面的频率

B．降低拍打水面的频率

C．无需拍打，A也会振动起来

D．无论怎样拍打，A都不会振动起来

Dad ______ from his paper and smiled when I entered the living room. [ ]

A. looked after

B. looked for

C. looked down

D. looked up