A:波长等于电磁波1s内出现的波峰数或波谷数
B:波长等于两个波峰(或波谷)之间的距离
C:波长等于相邻两个波峰(或波谷)之间的距离
D:波长等于1s内波长传播的距离
如图,AB⊥AC,AD⊥BC,已知AB=6,BC=9,则图中线段的长BD=____ ,AD=____ ,AC=____
—Do you know _________ boy under the tree?
— Yes, he knows a lot about dinosaurs though he is _________ 11-year-old boy.