Question

Choose a suitable adverb clause to complete the sentence:
Last night's episode was so breathtaking _______.

• A. but she was so sleepy
• B. and she went to sleep
• C. that she couldn't sleep after watching it
• D. None of the above

Answer 1

Answer: (C)

that she couldn't sleep after watching it

An adverb clause is a group of words containing a subject and a verb, and it functions as an adverb, by modifying an adjective, verb or an adverb. Adverb clauses generally begin with a subordinate conjunction (since, because, while, etc.)
Option C is correct because it is an adverb clause of result or consequence, "so...that" is a pair of conjunctions that refers to result: "so breathtaking that she couldn't sleep".
Option A is incorrect because it is a coordinate/independent clause, whereas an adverb clause is subordinate/dependent.
Similarly, option B is incorrect because it is a coordinate clause.
Option D is incorrect because the answer lies in C.