Chỉ ra các phân số tối giản trong các phân số sau:  1 3 ; − 2 5 ; 8 − 10 ; − 10 − 11 ; − 15 − 12 ; − 21 42

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

Many ants forage across the countryside in large numbers and undertake mass migrations; these activities proceed because one ant lays a trail on the ground for the others to follow. As a worker ant returns home after finding a source of food, it marks the route by intermittently touching its stinger to the ground and depositing a tiny amount of trail pheromone – a mixture of chemicals that delivers diverse messages as the context changes. These trails incorporate no directional information and may be followed by other ants in either direction.

Unlike some other messages, such as the one arising from a dead ant, a food trail has to be kept secret from members of other species. It is not surprising then that ant species use a wide variety of compounds as trail pheromones. Ants can be extremely sensitive to these signals. Investigators working with the trail pheromone of the leafcutter ant Atta texana calculated that one milligram of this substance would suffice to lead a column of ants three times around Earth.

The vapor of the evaporating pheromone over the trail guides an ant along the way, and the ant detects this signal with receptors in its antennae. A trail pheromone will evaporate to furnish the highest concentration of vapor right over the trail, in what is called a vapor space. In following the trail, the ant moves to the right and left, oscillating from side to side across the line of the trail itself, bringing first one and then the other antenna into the vapor space. As the ant moves to the right, its left antenna arrives in the vapor space.

The signal it receives causes it to swing to the left, and the ant then pursues this new course until its right antenna reaches the vapor space. It then swings back to the right, and so weaves back and forth down the trail.

The author mentions the trail pheromone of the leafcutter ant in paragraph 2 to point out

A. a type of ant that is common in many parts of the world

B. how little pheromone is needed to mark a trail

C. the different types of pheromones ants can produce

D. that certain ants can produce up to one milligram of pheromone

Mark the letter A, B, C, or D on your answer sheet to indicate the sentence that best completes each of the following exchanges.

Ann and Mary are studying in their classroom.

Ann: "Can I borrow you dictionary?"

Mary: "______"

A. I'm afraid I can't.

B. Here you are!

C. I think so

D. It doesn't matter.

combines each pair of sentences in the following questions.

Henry tasted the pleasures of modern city life. Then he found life in his village hard and unattractive.

A. Having tasted the pleasures of modern city life, Henry found life in his village hard and unattractive.

B. After Henry found life in his village hard and unattractive, he tasted the pleasures of modern city life.

C. If Henry had tasted the pleasures of modern city life, he would have found life in his village hard and unattractive.

D. Although Henry tasted the pleasures of modern city life, he found life in his village hard and unattractive.