6. Xiao Hu placed chess pieces on the grid points of a $19 \times 19$ Go board, first forming a solid rectangular array. Then, by adding 45 more chess pieces, he formed a larger solid rectangular array with one side unchanged. How many chess pieces did Xiao Hu use at most?
(A) 285
(B) 171
(C) 95
(D) 57
Analysis: For the newly formed solid dot matrix, its rows and columns must be divisors of 45. The divisors of 45 are $1,3,5,9,15,45$. Since the matrix is within a $19 \times 19$ grid, the divisors must be less than 19, so they can only be $3,15,5,9$. After verification, when the 45 dots are arranged horizontally in 3 rows, with 15 dots in each row, the unchanged side is 15. To maximize the rectangle, the original solid matrix has the other side as 16, so the total is $15 \times 19=285$.