Trường hợp nào sau đây không liên quan đến phản xạ âm

A. Trong các phòng hòa nhạc , người ta treo dèm nhung

B. Điều chỉnh âm thanh của của máy radio nhỏ lại

C. Dùng máy đo độ sâu của biển

D. Tiếng vang xảy ra trong các hang động

1.''we are living in london with our parents now ?'' the children said

- The children said .to be living in London with their parents now.....

2.the weather was cold , but they decided to go for a picnic

- Although ..the weather was cold, they decided to go for a picnic...

3the boys are very naughty . they live next door to us -

-The boys ..live the next door to us are very naughty...

4.the workers have built this building for 4 months -

-This ......building has been built for 4 months by the workers.....

- Vai trò của thú đối với đời sống con người:  

+ Làm thực phẩm

+Làm đồ mĩ nghệ

+Làm sức kéo

+Tiêu diệt gặm nhấm

+Cung cấp da, lông

+Làm thuốc

   Khi nóng thì không khí nở ra làm trọng lượng riêng giảm, còn khi lạnh, không khí co lại làm trọng lượng riêng tăng. Vì vậy, không khí nóng nhẹ hơn không khí lạnh.

\(7.25-x+5.48=3.2\)

\(175-x+240=6\)

\(175-x=6-240\)

\(175-x=-234\)

\(x=175+234\)

\(x=409\)

Vậy x=409

       Vietnam has many beautiful and attractive places, but the one I want to go to the most is Ha Long Bay in Quang Ninh. It has been recognized as a World Natural Heritage by Unesco. And it is also recognized as the most famous and beautiful natural heritage in Vietnam. It has many beautiful islands, especially Tuan Chau. Ha Long Bay has a vast and immense coastline. The sea is very beautiful. And there are deep caves. If there is a sea, there will certainly be boats: sailing ships, yatch,..... Not only that, the evening in Ha Long is also very wonderful. We can go to the amusement park and play a lot of games. The roller coaster, the spin, the light from the flashing lights, it's all great, isn't it? There are a lot of things that I want to explore in the Bay Ha Long. I want to sail around all the islands. And I want to venture, explore dark and terrifying caves. I also want to try the delicious and wonderful seafood dishes. I want to go to Ha Long even once in my life, to see the place recognized as the most beautiful place in Vietnam.