The person that I trust the most is my best friend, Giang. She is 18 years old. She is also my English private tutor who has been teaching me for 3 years.

She is very pretty with a lovely round face and beautiful dark brown eyes. Although she is not so tall, her black hair is very long and silky. Her favorite clothes are jeans, T-shirts and sneakers for their comfort. At first, she appeared serious but later she showed her humorous side. She always makes me laugh all the times when we see each other. She is not only good at English but she also plays piano exceptionally well. She can even compose songs. Furthermore, conscientiousness is the most beautiful personality of her. She makes people feel pleasant in her caring and understanding quality.

She is both well-qualified teacher and a precious friend to me. I hope our relationship can last long and remain stable.

Một lò xo có chiều dài tự nhiên 10 cm và có độ cứng 40 N/m. Giữ cố định một đầu và tác dụng vào đầu kia một lực 1 N để nén lò xo. Chiều dài của lò xo khi bị nén là

A. 5 cm.

B. 15 cm.

C. 10 cm.

D. 7,5 cm.

Nêu những khó khăn chính của các nước đang phát triển hiện nay.