I really want to tell you about my friend, one of my best friend is Giang. We were  classmate during our five years at  Da Nang University of Technology. He was a good student of my class. He has black hair and bright eyes and is very good – looking. Giang and I had many things in common, especially, our hobbies. We both like traveling, billiard, table tennis, reading book and singing.

He has a kind heart and is easy to get on with everybody. The deeper our mutual understanding becomes the more we feel closely attached to each other. We are the best pupils in our class. Giang is always at the top. He is good at every subject, but he never shows pride in his abilities and always tries to learn harder.

Giang’s family is not rich. His parents are farmers. He often spends most of his free time doing useful things to help his parents. I like Giang very much because of his honesty and straight forwardness. He is always ready to help his weak friends in their study; as a result, lots of friends, thanks to his whole-hearted help, have become good ones.

Đoạn mạch mắc nối tiếp theo thứ tự gồm điện trở R, cuộn dây có điện trở r và tụ điện có điện dung thay đổi được. Đặt vào hai đầu đoạn mạch một điện áp xoay chiều có tần số 50 Hz. Đồ thị hình bên mô tả mối quan hệ của điện áp hiệu dụng U_RLC giữa hai đầu đoạn mạch chứa cuộn dây và tụ điện theo điện dung. Điện trở r có giá trị gần nhất với giá trị nào sau đây ? 

A. 61 Ω

B. 81 Ω

C. 71 Ω

D. 91 Ω