Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

As a result of pollution, Lake Erie, on the borders of the USA and Canada, is now without any living things. Pollutions in water are not simply a matter of “poisons” killing large numbers of fish overnight. Very often the effects of pollution are not noticed for many months or years because the first organisms to be affected are either plants or plankton. But these organisms are the food of fish and birds and other creatures. When this food disappears, the fish and birds die in this way a whole food chain can be wiped out, and it is not until dead fish and water birds are seen at the river’s edge or on the sea shore that people realize what is happening. Where do the substances which pollute the water come from? There are two main sources – sewage and industrial waste. As more detergent is used in the home, so more of it is finally put into our rivers, lakes and seas.

Detergents harm water birds dissolving the natural substance which keep their feather waterproof. Sewage itself, if it is not properly treated, makes the water dirty and prevents all forms of life in rivers and the sea from receiving the oxygen they need. Industrial waste is even more harmful since there are many high poisonous things in it, such as copper and lead. So, if we want to stop this pollution, the answer is simple, sewage and industrial waste must be made clean before flowing into the water. It may already be too late to save some rivers and lakes, but others can still be saved if the correct action is taken at once.

According to the passage, the way to stop water pollution is ______.

A. to make the waste material harmless.

B. to realize the serious situation clearly

C. to make special room in the sea for our rubbish

D. to put oxygen into the river

Vẽ đồ thị của hai hàm số  y = 1 4 x 2  và  y = - 1 4 x 2  trên cùng một hệ trục tọa độ.

Tìm trên đồ thị của hàm số  điểm N có cùng hoành độ với M, điểm N’ có cùng hoành độ với M’. Đường thẳng NN’ có song song với Ox không? Vì sao? Tìm tung độ điểm N và N’ bằng hai cách:

- Ước lượng trên hình vẽ;

- Tính toán theo công thức.

Dựa vào đồ thị hàm số y = cos x, tìm các giá trị của x để cos x = 1/2

Tìm những số T sao cho f(x + T) với mọi x thuộc tập xác định của hàm số sau:

a) f(x) = sinx;

b) f(x) = tanx.

Để khử hết V lít xeton A mạch hở cần 3V lít H₂, phản ứng hoàn toàn thu được hợp chất B. Cho toàn bộ B tác dụng với Na dư thu được V lít khí H₂. Các khí đo ở cùng điều kiện. Công thức phân tử tổng quát của A là:

A.  C n H 2 n − 4 O 2

B.  C n H 2 n − 2 O

C.  C n H 2 n − 4 O

D.  C n H 2 n − 2 O 2

Your English is improving considerably. It is getting _________

A. better and better

B. well and better

C. good and better

D. good and well