Let $$S_1 = \left\{100, 105, 110, 115, ...\right\}$$ and $$S_2 = \left\{100, 95, 90, 85, ...\right\}$$ be two series inarithmetic progression. If $$a_k$$ and $$b_k$$ are the k-th terms of
$$S_1$$ and $$S_2$$, respectively, then $$\sum_{k=1}^{20}a_k b_k$$ equals ____________.

A

137275

B

138250

C

137225

D

135375