Explanation : 'Unless' denotes condition, 'until' denotes time. 'Choice (b) has adverb clause of time. With unless, another negative is not used. 'Unless ..... don't' is wrong. So, Choices (c) and (d) are incorrect. Hence correct choice is (a).
Explanation : Choices (a) and (b) state the same idea f(a) - passive voice; (b) Active voice] that the students followed the instructions, which differs from the question statement. Instructions cannot be advised. So, choice (c) is meaningless. Choice (d)
states the context of the given statement aptly.
Explanation : Despite means in spite of. Despite of is a wrong usage. 'His' need not be used in choice (c), as the sentence is about one person. Choice (d) means that the classes were ill. Hence correct choice is (b).
Explanation : Choice (a) best restates the given sentence, though choice (b) is the repetition of the given statement. 'Not one can deny' in choice (c) and for anyone to deny' in choice (d) make them incorrect. Hence correct choiceis (a).