4. In a box, there are several black and white chess pieces. If one black piece is taken out, the ratio of the remaining black pieces to white pieces is $9: 7$. If the black piece is put back, and then one white piece is taken out, the ratio of the remaining black pieces to white pieces is $7: 5$. How many more black pieces than white pieces were originally in the box?
A. 5
B. 6
C. 7
D. 8
Analysis: Ratio and proportion.
The key is to find the invariant, the total number of remaining pieces does not change after two operations.
Taking one black piece: Black: White: Remaining pieces sum = 9:7:16
Taking one white piece: Black: White: Remaining pieces sum $=7: 5: 12$
$$
[12,16]=48
$$
9:7:16=27:21:48, $7: 5: 12=28: 20: 48$, so originally there were 28 black pieces and 21 white pieces, so the number of black pieces is more than the number of white pieces by $28-21=7$. Choose C.