C. It's nice of you to say so. This is the most appropriate response to a compliment.

A. turn off. We turn off electrical appliances to stop them from working.

C. would buy. This is indirect speech (reported speech). The original question was likely "Will you buy...?" which changes to "would buy" in reported speech.

A. which. "Which" is used to introduce a non-defining relative clause, providing extra information about Hue.

C. But. "But" is the correct conjunction to show contrast between the desire to go and the obligation to stay.

C. to wait. "Mind" is followed by a gerund (verb + -ing). It should be "We don't mind waiting for you..."

B. careful. "Careful" is an adjective. We need an adverb to modify the verb "prepare." It should be "carefully."

B. was. The sentence uses "Although," which already indicates a contrast. "But" is redundant after "Although." The correct sentence is "Although he was tired, he still went to work yesterday."

A. at. It should be "in Ho Chi Minh City." We use "in" for cities.

D. still slept. The sentence describes two actions happening at the same time in the past. We should use the past continuous tense for the action that was in progress when the other action happened. It should be "I was still sleeping."