Question 1:
The correct answer is the first sentence because "who lives across the street" is a restrictive relative clause. Restrictive relative clauses, also known as defining relative clauses, post-modify a noun and they provide essential information that cannot be omitted.
Question 2:
The correct answer is the third sentence because "famous for the thousands of libraries he built" is adding new information about the subject [Andrew Carnegie] which is additional to the main clause.
Question 3:
The correct answer is the first sentence because "which was built at the turn of the century" is a non-restrictive relative clause. Non-restrictive relative clauses, also known as non-defining relative clauses, provide additional information about the noun they post-modify, but this information is not essential. Therefore, non-restrictive relative clauses can be omitted.
Question 4:
The correct answer is the third sentence because "who think raising children is easy" is a restrictive relative clause. Restrictive relative clauses, also known as defining relative clauses, post-modify a noun and they provide essential information that cannot be omitted.
Question 5:
The correct answer is the third sentence because "who graduated first in her class" is a non-restrictive relative clause. Non-restrictive relative clauses, also known as non-defining relative clauses, provide additional information about the noun they post-modify, but this information is not essential. Therefore, non-restrictive relative clauses can be omitted.
Question 6:
The correct answer is the third sentence because "which stays open twenty-four hours" is a non-restrictive relative clause. Non-restrictive relative clauses, also known as non-defining relative clauses, provide additional information about the noun they post-modify, but this information is not essential. Therefore, non-restrictive relative clauses can be omitted.
Question 7:
The correct answer is the second sentence because "which dropped the atomic bomb on Hiroshima" is a non-restrictive relative clause. Non-restrictive relative clauses, also known as non-defining relative clauses, provide additional information about the noun they post-modify, but this information is not essential. Therefore, non-restrictive relative clauses can be omitted.
Question 8:
The correct answer is the first sentence because "who used to work for the post office" is a non-restrictive relative clause. Non-restrictive relative clauses, also known as non-defining relative clauses, provide additional information about the noun they post-modify, but this information is not essential. Therefore, non-restrictive relative clauses can be omitted.
Question 9:
The correct answer is the first sentence because no comma should separate the verb and its direct object nor the adjectives which are not of the same category.
Question 10:
The correct answer is the third sentence because an If-clause is an adverbial clause. Adverbial clauses are dependent clauses of the main independent clause, which means that, when they come at the beginning of a sentence, they should have a comma to signal their dependency to the main clause.