7. On June 9, 1898, Britain forced the Qing government to sign a treaty leasing 975.1 square kilometers of Hong Kong land to Britain for 99 years. On July 1, 1997, it was known that it was a Tuesday. Therefore, June 9, 1898, was a ( ).
(A) Tuesday
(B) Wednesday
(C) Thursday
(D) Friday
(Note: In the Gregorian calendar, any year that is a multiple of 4 but not a multiple of 100, or a multiple of 400, is a leap year. In a leap year, February has 29 days; otherwise, February has 28 days.)
7.C.

Solution 1: $\because 365 \equiv 1(\bmod 7)$,
$\therefore$ If $S$ represents the total number of days from the beginning of the year to the required day, $x$ represents the year, and $y$ represents the day of the year, then we have
$$
\begin{array}{l} 
S \equiv(x-1)+\left[\frac{x-1}{4}\right]-\left[\frac{x-1}{100}\right]+\left[\frac{x-1}{400}\right] \\
+y(\bmod 7) . \\
\text { When } x=1898, y=31 \times 3+30+28+9=160 \text {, } \\
S \equiv 1897+\left[\frac{1897}{4}\right]-\left[\frac{1897}{100}\right]+\left[\frac{1897}{400}\right]+160 \\
\equiv 1897+474-18+4+160 \\
\equiv 2517 \equiv 4(\bmod 7) .
\end{array}
$$

Solution 2: It is known that July 1, 1997, was a Tuesday, so we can deduce that June 9, 1997, was a ... and from June 9, 1898, to June 9, 1997, there are 99 years. Among these, there are 24 leap years, so we know that $99 \times 365$ $+24 \equiv 99+24 \equiv 4(\bmod 7) \cdot 1-4 \equiv-3 \equiv 4(\bmod 7)$.