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答案：D
解析：
At first, there are n students in the class, and 6 of them are seniors. It follows that initially n-6 students are not seniors. If 7 juniors are added to the class, there will be n-6 + 7, or n+1, students in the class who are not seniors.
adj. 许多，多种多样的
n. 倍数，并联
