Một sóng điện từ lan truyền trong chân không dọc theo đường thẳng từ điểm M đến điểm N cách nhau $45 m$. Biết sóng này có thành phần điện trường tại mỗi điểm biến thiên điều hòa theo thời gian với tần số $5 MHz$. Lấy $c = 3.10^8m/s$. Ở thời điểm t, cường độ điện trường tại M bằng $0$. Thời điểm nào sau đây cường độ điện trường tại N bằng $0$?

​Bài tập 2: Chọn từ hoặc cụm từ thích hợp hoàn thành những câu sau.

1. He...............................for 25 years before he retired last year.

​A. has taught​B. had been teaching​C. has been teaching​D. was teaching

2. Please send me a postcard as soon as you .......................... in London.

​A. will arrive​B. is going to arrive​C. have arrived​D. arrive

3. I saw John yesterday morning while I.............................home from work.

​A. walked​B. am walking​C. was walking​D. had been walking

​4. As soon as you........................your homework, you will be allowed to go out.

​A. are doing​B. had done C. did D. have done

5. After Mary..............her degree, she intends to work in her father’s company.

​A. will finish​B. finishesn C. is finishing D. will have finished

​6. She has worked as a secretary...........................she graduated fro college.

​A. until​B. while​C. before​D. since

7. Ann ............................for me when I arrived

​A. was waiting​B. waited​C. had waited C. has been waiting

​8. My handbag was stolen...........................we were playing tennis.

​A. after​B. before​C. during​D. while

9. They won’t come home until they.....................everything.

​A. had seen​B. saw​C. were seeing​D. see

​10. Don’t come..........................I have finished lunch.

​A. after​B. as soon as​C. since​D. until