4. Give the corresponding active construction: I. Many interesting experiments are carried out in our laboratory /scientists/ 2. She hopes she will not be refused a visa /the Embassy/. 3. Our room has been papered lately/the decorators/. 4. When I came the last stedent was being examined /the professor/. 5. Monday was agreed upon as the most suitable day for the first rehearsal /the actors/ 6. Problems of vital importance are touched upon in this book. 7. Mothers and children in our country are taken much care of /the government. 8. The document has been signed by the president of the toard. 9. This poem was written by an unknown author.

Sure! Here is a detailed and step-by-step explanation of the corresponding active construction for each sentence:

1. Many scientists carry out many interesting experiments in our laboratory.
- To change the sentence from a passive construction to an active one, we need to identify the subject (who is performing the action) and make it the doer of the action. In this case, the subject is "scientists." So, we rewrite the sentence using the active voice and the subject as the doer of the action: "Many scientists carry out many interesting experiments in our laboratory."

2. The Embassy refuses her a visa.
- Again, we identify the subject (who is performing the action) and make it the doer of the action. In this case, the subject is "the Embassy." So, we rewrite the sentence: "The Embassy refuses her a visa."

3. The decorators have recently papered our room.
- Here, the subject is "the decorators." We rewrite the sentence: "The decorators have recently papered our room."

4. The professor was examining the last student when I came.
- The subject here is "the professor." We rewrite the sentence: "The professor was examining the last student when I came."

5. The actors have agreed upon Monday as the most suitable day for the first rehearsal.
- The subject is "the actors." We rewrite the sentence: "The actors have agreed upon Monday as the most suitable day for the first rehearsal."

6. This book touches upon problems of vital importance.
- Here, the subject is "this book." We rewrite the sentence: "This book touches upon problems of vital importance."

7. The government takes much care of mothers and children in our country.
- The subject in this sentence is "the government." We rewrite the sentence: "The government takes much care of mothers and children in our country."

8. The president of the board has signed the document.
- The subject here is "the president of the board." We rewrite the sentence: "The president of the board has signed the document."

9. An unknown author wrote this poem.
- The subject is "an unknown author." We rewrite the sentence: "An unknown author wrote this poem."

I hope this detailed explanation helps you understand the corresponding active constructions for each sentence.