7. Suppose $a_{n}$ denotes the last two digits of $7^{n}$. For example, $a_{2}=49, a_{3}=43$. Find the value of $a_{1}+a_{2}+a_{3}+$ $\qquad$ $+a_{2007}$
(A) 50189
(B) 50199
(C) 50209
(D) 50219
(E) 50229
(B)

Observe that $a_{n}$ repeats itself as shown in the following table.
\begin{tabular}{|l|l|l|l|l|l|l|l|l|}
\hline$n$ & 1 & 2 & 3 & 4 & 5 & 6 & 7 & $\ldots \ldots . .$. \\
\hline$a_{n}$ & 07 & 49 & 43 & 01 & 07 & 49 & 43 & $\ldots . . .$. \\
\hline
\end{tabular}

Further, $a_{1}+a_{2}+a_{3}+a_{4}-100$.
Notice that $2007=4 \times 501+3$.
Hence the sum equals $501 \times 100+7+49+43=50199$.