The passive voice is used if you don't know who performed the action described in the sentence.
Correct.
Covid-19 blood samples have been stolen from a laboratory in India by monkeys in the last months
TRUE
Today, paper (USE) for hundreds of things - not only books and newspapers, but also money, stamps, bags and even clothes.
is used
In Victorian times, people considered it bad luck to sign a Valentine's Day card.
TRUE
In ancient times, before the invention of paper, people (WRITE) on animal skins, bones and stones. In 2700 BC the Egyptians (START) to make papyrus, which was similar to paper.
wrote, started
People exchange about 1 billion Valentine's Day cards each year.
TRUE
But the first real form of paper (INVENT) in 105 AD by a Chinese government official. It (MAKE) from a mixture of plants and clothes. The Chinese (KEEP) their new discovery a secret for many centuries.
was invented, was made, kept
By the end of next month, a 1000-word essay will have been written by me.
By the end of next month, I will have written a 1000-word essay.
The current American flag was designed by a high school student
CORRECT
There’s only one letter that doesn’t appear in any U.S. state name.
TRUE
You’ll find a Z (Arizona), a J (New Jersey), and even two X’s (New Mexico and Texas)—but not a single Q.
I had sent the parcel in the morning. (Active) / The parcel ……………….. in the morning. (Passive)
had been sent
He thinks that someone is teasing Allie.
The passive voice of this sentence should be "He thinks that Allie is teased."
Wrong
"He thinks that Allie is being teased."
That tiny pocket in jeans was designed to store pocket watches
TRUE
We will have finished the work by Monday. (Active) / The work ………………… by Monday. (Passive)
will have been finished