Trong các làng nghề chế tạo đá mỹ nghệ, người ta làm được nhiều đồ trang sức bằng đá rất đẹp. Một người thợ thủ công chế tạo vòng đeo tay hình tròn bằng đá xanh, biết rằng bán kính vòng ngoài của vòng là \(7\,{\rm{cm}}\), bán kính vòng trong là \(5\,{\rm{cm}}\) (hình vẽ). Tính thể tích của chiếc vòng (theo đơn vị đo là centimét khối và làm tròn kết quả đến hàng đơn vị).

Biện pháp đầu tiên được giới thiệu để cải tạo đất chua là:

Với mỗi môi trường pH khác nhau, các amino acid tồn tại ở các dạng như sau:

Phát biểu nào sau đây không chính xác?

Trong không gian $O x y z$, cho hai điểm $A(2 ; 3 ;-1)$ và $B(-4 ; 1 ; 9)$. Trung điểm $I$ của đoạn thẳng $A B$ có tọa độ là

Cho hàm số $y=f(x)$ xác định, liên tục trên $\mathbb{R}$ và có bảng biến thiên sau:

Khẳng định nào sau đây đúng?

Read the following advertisement and mark the letter A, B, C or D on your answer sheet to indicate the option that best fits each of the numbered blanks from 1 to 6.

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Mặt bằng của ngôi nhà:

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the best answer to each of the following questions from 31 to 40.

Biting into a ripe, juicy fruit brings a smile to many people’s faces. But eating that same fruit a week before it is ripe probably wouldn’t be as enjoyable! Unripe fruits are often hard and bitter, but that serves a purpose. The hard exterior of an unripe fruit protects the developing seed inside. After a seed fully develops inside the fruit, the fruit changes color, softens, and gets sweeter to become more attractive to animals and humans. These organisms eat the fruit and spread its seeds, ensuring the plant’s survival and regrowth.

But how does a fruit ripen? The ripening process involves many complex chemical reactions. When a fruit ripens, it goes through a chemical change. A chemical change happens when bonds within molecules break, and the atoms that once formed them regroup into new molecules. Chemical changes are irreversible. Once a fruit ripens, it cannot become unripe again.

One molecule responsible for this chemical change is ethylene. Ethylene (C2H4) is a colorless gas made up of carbon and hydrogen atoms. (I). It is a naturally occurring molecule in the atmosphere. (II). It is also an important plant hormone. (III). Plants release ethylene through the growing tips of roots, flowers, and ripening fruit. (IV). Although ethylene serves other purposes in plants, one of its main purposes is to trigger the chemical reactions that cause fruits to ripen. As the fruit matures, oxygen in the air helps stimulate the production of ethylene. This ethylene production is a signal that begins fruit ripening. For that reason, unripe fruits have low levels of ethylene, while ripe fruits have higher levels of ethylene. Most fruits produce ethylene and respond to ethylene in the atmosphere.

Although fruits require ethylene to ripen, different types of fruits respond to this molecule in two distinct ways: non-climacteric fruits and climacteric fruits. Non-climacteric fruits do not ripen after harvesting. They produce very little ethylene of their own. Climacteric fruits, on the other hand, continue to ripen after they are picked. These fruits produce a large amount of ethylene. Climacteric fruits can also speed up the ripening process of other fruits nearby since they release high levels of ethylene into the air.

Which of the following is OPPOSITE in meaning to exterior in paragraph 1?

Chọn ý đúng: Loại bào quan nào sau đây được gọi là túi tự tử?

Trong sóng cơ, tốc độ truyền sóng là

Đâu là trào lưu nghệ thuật dân gian được hình thành ở Việt Nam từ thế kỉ XVI đến XVIII?