a) Example: We carry out many experiments in our laboratory.

Many experiments are carried out by us in our laboratory.

1. The secretary sent all the mail off an hour ago. 2. The doctor examined the patient and wrote out a prescription. 3. The students take written exams at the end of the term. 4. Columbus discovered America in 1492. 5. We often change our plans. 6. Shakespeare wrote 37 plays.

b) Example: He was given an interesting problem to solve. (the teacher)

The teacher gave him an interesting problem to solve.

1. The students were warmly thanked for the help (The Dean). 2. He was offered many books on the subject of his report. (a librarian) 3. The children were sent to the sea for summer. (the parents) 4. The questions were asked at the police station. (a police officer) 5. The experiment is discussed every day. (the scientist) 6. The delegates were interviewed near the hotel. (the newspaper reporters)

I Мне кажется все варианты А кроме 4 (не знаю) ведь Present perfect - настоящее совершенное время, обозначает действие, которое совершено в но его результат связан с настоящим.
II
1. Has he travelled the world? He hasn’t travelled the world.
2. Has Margaret gone to Thailand? Margaret hasn’t gone to Thailand.
3. Have we seen famous sights? We havn’t seen famous sights.
4. Has she bought souvenirs for her friends and family? She hasn’t bought souvenirs for her friends and family.
5. Have students been here since 8 o’clock? Students havn’t been here since 8 o’clock.  
III Какие слова выделенные?
IV Тут могу ошибаться:
1. с возможно в
2. в точно не с
3. с
4.с
5. c точно не в
6. а
7. с
8. с точно не в
9. в
10. а
11. в
12. а
13. с
14. с
15. в

1. I did not see my friend Mike last summer. Actually, I did not see him since we leftschool. 
2. You saw Liza in the last few days? – No, but I have seen her today. We are going outfor dinner.
3. You ever was to South America? – Yes, I‟ve been there several times. – Whenyou last went there? – A couple of months ago. I sometimes go there on business.
 4. Elmer took hundreds of pictures when he was in Europe.
5. Helen was here earlier but I think now she already left. She usually leaves theoffice at 5 o'clock.
6.  Patrick left the party early because he was sick.
7. –You ever ride a camel? – Yes, I rode a camel when I was in Egypt last year.
8. Erick had a lot of free time since he moved to Thailand.
9. Bob visited the White House when he was in Washington.
10. I was to the supermarket. Look, Ive bought a lot of things.

1. Alice visited more countries than she can count.
2. I live  in my hometown all my life.
3. We saw many interesting sights in the last few days.
4. Kate no found a new job yet.
5.  Molly has eaten three pieces of cake today.