Our school is on Guangming Street. It's next to the People's Library. There is a park across from our school. The pay phone is between our school and a post office. Our school is very beautiful. There are many trees and flowers in it. If you want to come to our school from the airport, take a taxi and go down Center Avenue. You can see a big restaurant on your right. There is a bridge near the restaurant. Go across it and turn left. Then go down Bridge Street. Turn left at New Park and go down Guangming Street. Our school is across from a bank.

这一天北极的昼夜长短情况是（　　）

试回答：

某物体发出的声音在空气中的波长为1 m，波速为340 m/s，在海水中的波长为4.5 m，此物体在海面上发出的声音经0.5 s听到回声，则海水深为____ m．