lắng nghe ý kiến của mọi người và sẵn sàng tranh luận vì lẽ phải

Một cái thước  A B = 1 m đặt trên mặt bàn nhẵn nằm ngang, có trục quay O cách đầu A một khoảng 80 cm (Hình 18.5). Một lực  F 1 = 4 N tác dụng lên đầu A theo phương vuông góc với thước và lực thứ hai F 2  tác dụng lên đầu B của thước và theo phương vuông góc với thước (không vẽ trên hình). Các lực đều nằm trên mặt phẳng nằm ngang. Nếu thước không chuyển động, thì lực tác dụng của trục quay O lên thước có hướng và độ lớn       

A. cùng hướng với  F 1 →  và có độ lớn  R = 20   N

B. cùng hướng với  F 1 →  và có độ lớn  R = 12   N

C. cùng hướng với  F 1 →  và có độ lớn  R = 16   N

D. ngược hướng với   F 1 → và có độ lớn  R = 20   N

Ancol no, đơn chức có 10 nguyên tử H trong phân tử có số đồng phân là

A. 5

B. 3

C. 4

D. 2

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.

"Did you see that?" Joe said to his friend Bill. "You're a great shooter!"

Bill caught the basketball and bounced it before throwing it again. The ball flew into the net.

"Bill, you never miss!" Joe said admiringly.

"Unless I'm in a real game," Bill complained. "Then I miss all the time."

Joe knew that Bill was right. Bill performed much better when he was having fun with Joe in the school yard than he did when he was playing for the school team in front of a large crowd.

"Maybe you just need to practice more," Joe suggested.

"But I practice all the time with you!" Bill objected. He shook his head. "I just can't play well when people are watching me."

"You play well when I'm watching," Joe pointed out.

"That's because I've known you since we were five years old," Bill said with a smile. "I'm just not comfortable playing when other people are around."

Joe nodded and understood, but he also had an idea.

The next day Joe and Bill met in the school yard again to practice. After a few minutes, Joe excused himself.

Practice without me," Joe said to his friend. "I'll be back in a minute."

Joe hurried through the school building, gathering together whomever he could find—two students, a math teacher, two secretaries, and a janitor. When Joe explained why he needed them, everyone was happy to help.

Joe reminded the group to stay quiet as they all went toward the school's basketball court. As Joe had hoped, Bill was still practicing basketball. He made five baskets in a row without noticing the silent people standing behind him.

"Hey, Bill!" Joe called out finally.

Bill turned. A look of surprise came over his face. "I just wanted to show you that you could play well with people watching you," Joe said. "Now you'll have nothing to worry about for the next game!" 

Why does the group have to be quiet when they go to the basketball court?

A. Because they do not want Bill to know they were there  

B. Because the group needs to listen to Joe’s instructions  

C. Because Joe is telling Bill what to do  

D. Because Bill likes to practice alone