Rewrite the sentences in the passive. Do not include the agent.
1 Companies make boxes out of cardboard because it’s a cheap material.
Boxes .
2 Builders won’t use concrete as much in the future.
Concrete .
3 A company called Dupont developed nylon in the 1930s.
Nylon .
4 The builders have broken the glass window.
The glass window .
5 People will recycle plastic more in the future.
Plastic .
6 Someone discovered rubber many years ago in South America.
Rubber .
7 People burn wood to heat buildings.
Wood .

1 Boxes are made out of cardboard because it's a cheap material.
Explanation: In this sentence, we want to rewrite it in the passive voice. The subject "companies" becomes the object in the passive voice. The verb "make" becomes "are made" in the passive voice. The prepositional phrase "out of cardboard" remains the same. The reason "because it's a cheap material" is not included in the passive voice.

2 Concrete won't be used as much in the future.
Explanation: Again, we want to rewrite the sentence in the passive voice. The subject "builders" becomes the object in the passive voice. The verb "won't use" becomes "won't be used" in the passive voice. The reason "in the future" is not included.

3 Nylon was developed by a company called Dupont in the 1930s.
Explanation: Here, the subject "a company called Dupont" becomes the object in the passive voice. The verb "developed" becomes "was developed" in the passive voice. The time reference "in the 1930s" remains the same.

4 The glass window has been broken by the builders.
Explanation: To rewrite this sentence in the passive voice, the subject "the builders" becomes the object in the passive voice. The verb "have broken" becomes "has been broken" in the passive voice. In the passive voice, the agent (the builders) is not included.

5 Plastic will be recycled more in the future.
Explanation: In this sentence, the subject "people" becomes the object in the passive voice. The verb "will recycle" becomes "will be recycled" in the passive voice. The reason "in the future" remains the same.

6 Rubber was discovered many years ago in South America.
Explanation: Here, the subject "someone" becomes the object in the passive voice. The verb "discovered" becomes "was discovered" in the passive voice. The time reference "many years ago" remains the same.

7 Wood is burned to heat buildings.
Explanation: To rewrite this sentence in the passive voice, the subject "people" becomes the object in the passive voice. The verb "burn" becomes "is burned" in the passive voice. In the passive voice, the agent (people) is not included.