Có hai bình không nhãn đựng riêng biệt hai loại chất lỏng: dầu bôi trơn máy, dầu thực vật. Cách đơn giản nhất để phân biệt hai chất lỏng trên là phương án nào dưới đây ? 

A. Cho dung dịch KOH dư vào

B. Cho Cu(OH)₂ vào

C. Đun nóng với dung dịch NaOH dư

D. Đun nóng với dung dịch KOH dư, đề nguội, rồi cho thêm từng giọt dung dịch CuSO₄

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 25 to 29. Fill in the appropriate word in question 28

Environmental pollution

Environmental pollution is a term that refers to all the ways by which man pollutes his surroundings. Man dirties the air with gases and smoke, (25) _______the water with chemicals and other substances, and damages the soil with (26) _______ many fertilizers and pesticides. Man also pollutes his surroundings in various other ways. For example, people ruin natural beauty by (27) _______ junk and litter on the land and in the water. They operate machines and motor vehicles that fill the air with disturbing noise.

Environmental pollution is one of the most serious propblems facing humankind today. Air, water, and soil are necessary to the survival of all living things. Badly polluted air can cause illness, and (28) _______ death.Polluted water kills fish and other (29) _______ life. Pollution of soil reduces the amount of land that is available for growing food.

A. therefore

B. consequently

C. even

D. just

n³-n=n(n²-1)=(n-1)n(n+1)

trong 3 số tự nhiên liên tiếp sẽ có 1 số chia hết cho 3

=>(n-1)n(n+1) chia hết cho 3

trong 2 số tự nhiên liên tiếp sẽ có 1 số chia hết cho 2

=>(n-1)n(n+1) chia hết cho 2

Vì (2;3)=1=>n³-n chia hết cho 6

=>đpcm