Chất dùng để làm khô khí  Cl 2 ẩm là

A.  H 2 SO 4 đặc

B.  Na 2 SO 3 khan

C. CaO

D. NaOH đặc

Ở một loài thực vật, alen A quy định hoa đỏ, alen a quy định hoa vàng, a1 quy định hoa trắng. Biết rằng alen A trội hoàn toàn so với alen a và a1, alen a trội hoàn toàn so với alen a1. Cho một cây hoa đỏ giao phấn với một cây hoa vàng thu được F1 . Theo lí thuyết, bao nhiêu tỉ lệ kiểu hình thu được ở F1 có thể có trong các trường hợp sau đây ?

(1) 100% đỏ.

(2) 75% đỏ : 25% vàng.

(3) 50% đỏ: 50% vàng.

(4) 50% đỏ : 50% trắng.

(5) 50% đỏ : 25% vàng : 25% trắng

A. 1

B.3

C.2

D.4

Hãy nêu những điểm giống nhau và khác nhau của hai sông Hoàng Hà và Trường Giang?

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 from 35 to 42.

Humans have struggled against weeds since the beginnings of agriculture. Marring our gardens is one of the milder effects of weeds - any plants that thrive where they are unwanted. They clog waterways, destroy wildlife habitats, and impede farming. Their spread eliminates grazing areas and accounts for one-third of all crop loss. They compete for sunlight, nutrients, and water with useful plants.

The global need for weed control had been answered mainly by the chemical industry. Its herbicides are effective and sometimes necessary, but some pose serious problems, particularly if misused. Toxic compounds threaten animal and public health when they accumulate in food plants, groundwater, and drinking water. They also harm workers who apply them.

In recent years, the chemical industry has introduced several herbicides that are more ecologically sound. Yet new chemicals alone cannot solve the world's weed problems. Hence, scientists are exploring the innate weed-killing powers of living organisms, primarily insects and microorganisms.

The biological agents now in use are environmentally benign and are harmless to humans. They can be chosen for their ability to attack selected targets and leave crops and other plants untouched. In contrast, some of the most effective chemicals kill virtually all the plants they come in contact with, sparing only those that are naturally resistant or have been genetically modified for resistance. Furthermore, a number of biological agents can be administered only once, after which no added applications are needed. Chemicals typically must be used several times per growing season.

The word "innate" in paragraph 3 is closest in meaning to __________. 

A. effective 

B. organic 

C. natural 

D. active 

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 from 35 to 42.

Humans have struggled against weeds since the beginnings of agriculture. Marring our gardens is one of the milder effects of weeds - any plants that thrive where they are unwanted. They clog waterways, destroy wildlife habitats, and impede farming. Their spread eliminates grazing areas and accounts for one-third of all crop loss. They compete for sunlight, nutrients, and water with useful plants.

The global need for weed control had been answered mainly by the chemical industry. Its herbicides are effective and sometimes necessary, but some pose serious problems, particularly if misused. Toxic compounds threaten animal and public health when they accumulate in food plants, groundwater, and drinking water. They also harm workers who apply them.

In recent years, the chemical industry has introduced several herbicides that are more ecologically sound. Yet new chemicals alone cannot solve the world's weed problems. Hence, scientists are exploring the innate weed-killing powers of living organisms, primarily insects and microorganisms.

The biological agents now in use are environmentally benign and are harmless to humans. They can be chosen for their ability to attack selected targets and leave crops and other plants untouched. In contrast, some of the most effective chemicals kill virtually all the plants they come in contact with, sparing only those that are naturally resistant or have been genetically modified for resistance. Furthermore, a number of biological agents can be administered only once, after which no added applications are needed. Chemicals typically must be used several times per growing season.

Which of the following is NOT given as an advantage of using biological agents over chemical herbicides? 

A. They are safer for workers. 

B. They are less likely to destroy desirable plants. 

C. They are more easily available. 

D. They do not have to be used as often.