Could you correct these grammar questions on non-defining relative clauses?

Hi teachers,
Do you think there are enough questions to introduce non-defining relative clauses? Will you add others?
Which one do you think is better for the purpose at the end of the exercise, 7 or 8?

Circle the letter that best completes the sentences.

1. A clause is
a. a group of words than can have more than one subject or verb.
b. a group of words tha has only one subjet or verb.

2. Dependent clauses
a. can stand alone as a sentence.
b. cannot stand alone as a sentence.

3. Independent clauses
a. can stand alone as a sentence.
b. cannot stand alone as a sentence.

4. Non-defining relative clauses are always
a. dependent clauses.
b. independent clauses.

5. Main clauses are always
a. dependent clauses.
b. independent clauses.

6. Relative clauses are also
a. adjective clauses that immediately follow the noun they modify.
a. adverb clauses that immediately follow the noun they modify.

7. Non-defining relative clauses can begin with
a. the relative pronouns who or which.
a. the question words who or which.

8. who or which in non-defining relative clauses are
a. relative pronouns
b. question words.

Thanks in advance.