Hai nguồn phát sóng có cùng tần số, nằm tại hai điểm S1 và S2. Tại các điểm nằm trên đường trung trực của S1S2 sẽ luôn luôn có cực đại giao thoa nếu hiệu số pha dao động của hai nguồn bằng

A.  π / 2

B. π

C.  3 π / 2

D.  2 π

The overcrowded population problems is one of the most problems in my country. It took hundreds of thousands of years for the world population to grow to 1 billion. However, in just another two centuries, it grew sevenfold. You know, in 2011, the total population of the world reached 7 billion. And today, it's about 7.5 billion. Tokyo, with 37.8 million residents, continues to be the largest city in the world in terms of population.

Advances in modern medicine are the main cause of this dramatic growth in the world population. It helps the birth rates increase and the death rates decline. The second contributor is the improvement in living standards. This leads to the rise of global lifespan, from 64.8 years in the early 1990s to 70.0 years today. Last but not least, the number of women of reproductive age is increasing day by day.

However, the rapid population growth can lead to the shortage in medical and educational facilities and services. The population grows fast so the demand for energy also increases.Therefore, energy shortages are also a big problem. In addition, air, water and land pollution caused by growing population in big cities has bad effects on people's health.

- Cơ thể người gồm 3 phần: + Đầu

+ Thân

+ Tay và chân

- Phần khoang ngực chứa tim và phổi

- Phần khoang bụng chứa dạ dày, ruột, gan, thận, bóng đái và các cơ quan sinh sản