This question is about understanding the use of the passive voice in English grammar. The correct answer is 'C. To hide the subject' because the passive voice is often used to focus on the action rather than the actor, effectively hiding the subject. Option A is incorrect because the active voice is typically used to emphasize the subject. Option B is incorrect because the passive voice actually de-emphasizes the action. Option D is incorrect because the future tense is used to describe future actions, not the passive voice. The main point is to understand the function of the passive voice in sentence construction.