Mọi người ơi giúp em chọn đáp án đúng kèm giải thích chi tiết lý do vì sao chọn giúp em với ạ! Em cảm ơn rất nhiều ạ.

11. I'm grateful that you helped my brother do that work.
A. Thank you for helping my brother do that work.
B. Thank you about helping my brother do that work.
C. Thank you because you helped me do that work.
D. Thank you that you help my brother do that work
12. I find it impossible not to worry about Lan.
A. I can't help feeling worried about Lan
B. Ifeel worried about Lan, but I can't do anything to help
C.I can't do nothing to help Lan although I feel worried about her
D. I'm unable to worry about Lan
13. You'd better take a taxi; it was raining hard outside
A. You'd better take a taxi in case it was raining
B. She advised us to take a tad because it was raining outside
C. She gave us a lift because it was raining heavily
D. She didn't lend us any raincoats but a taxi
14. My brother and I go to that school
A. I went to that school and my brother, too
B. I went to that school and so my brother did
C. I went to that school and so did my brother
D. I went to that school and so my brother did, too
15. The meeting was put off because of pressure of time
A. There was not enough time to hold the meeting
B. People wanted to get away, so the meeting began early
C. The meeting was planned to start in a short time
D. The meeting lasted much longer than usual
16. When they arrived, the good seats were already taken
A. They didn't get good seats because they arrived too late
B. They arrived late enough to get good seats
C. They had to stand for the whole show
D. We couldn't get good seats although we arrived early
17. They would never have accepted his money if they had known his plans
A. They knew what he wanted to do, so they refused his money
B. They agreed with his wishes because they were glad to have his money
C. They didn't know his plans and never took money from him
D. They took the money be offered them without realizing
18. He could not afford to buy the car
A. He couldn't buy the car because it was too expensive
B. He bought the car because it was expensive
C. He bought the car though it was expensive
D. He had enough money but he didn't buy the car
19. Tam is not good at English and neither am I
A. Neither Tam nor I am good at English
B. Either Tam or I am good at English
C. I'm not as good at English as Tam is
D. Tam isn't good at English but I am
20. This is the first time I attend such an exciting holiday.
A. The first holiday I attended was exciting.
B. I had the first exciting holiday.
C. My attendance at the first holiday was exciting.
D. I have never attended such an exciting holiday before.

Chọn đáp án và giải thích tại sao lại chọn đáp án đó ạ. Em cảm ơn ạ 

Mọi người giải thích giúp em câu này với được không ạ. Em đã tìm ra được điều kiện xác định là  .Nhưng khoảng từ pi/2 đến -pi/4  thì vẫn bao gồm đáp án A mà sao mình lại chọn ạ