7. Let $x, y$ be real numbers, and satisfy
$$
\left\{\begin{array}{l}
(x-1)^{3}+1998(x-1)=\cdots 1, \\
(y-1)^{3}+1998(y-1)=1 .
\end{array}\right.
$$

Then $x+y=(\quad)$.
(A) 1
(B) -1
(C) 2
(D) -2
7.C

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However, it seems there was a misunderstanding in your request. The text "7.C" does not require translation as it is already in a form that is the same in both Chinese and English. If you have more text to translate or need further assistance, feel free to let me know!