Một con lắc lò xo có độ cứng 100 N/m và vật nhỏ có khối lượng m. Tác dụng lên vật ngoại lực F   =   20 cos 10 π t   ( N ) (t tính bằng s) dọc theo trục lò xo thì xảy ra hiện tượng cộng hưởng. Lấy π 2 = 10 . Giá trị của m là

A. 0,4 kg.

B. 1 kg.

C. 250 g.

D. 100 g.

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

Child workers, some as young as 10, have been found working in a textile factory in conditions described as close to slavery to produce clothes that appear destined for one of the major high street retailers. The discovery of these children working in appalling conditions in the Shahpur Jat area of Delhi has renewed concerns about the (15)______ by some large retail chains of their garment production to India, recognised (16)______ the United Nations as one of the world’s hotspots for child labour. According to one (17)______, over 20 per cent of India's economy is dependent on children, (18)______ comes to a total of 55 million youngsters under 14 working. Consumers in the West should not only be demanding answers from retailers about how their (19)______ are produced but also should be looking into their consciences at how they spend their money and whether cheap prices in the West are worth the suffering caused to so many children.

Điền vào ô 18

A. who

B. which

C. what

D. that