6. There is a certain amount of saltwater with a concentration of $3 \%$. After adding $a$ grams of pure water, the concentration of the saltwater becomes $2 \%$. After adding another $a$ grams of pure water, the concentration of the saltwater at this point is ( ).
(A) $1 \%$
(B) $1.25 \%$
(C) $1.5 \%$
(D) $1.75 \%$
6. C.

Let's assume that in a 3% salt solution, there are 97 grams of water and 3 grams of salt. Therefore, adding 50 grams of water can turn it into a 2% salt solution, and then adding another 50 grams of water can turn it into a 1.5% salt solution.