【Answer】D
【Analysis】The largest is 4321, which is the 24th, so 4312 is the 23rd, 4231 is the 22nd, and 4213 is the 21st. Therefore, the difference between the 23rd and the 21st is 4312-4213=99
2、Arrange all four-digit numbers formed by using $1, 2, 3, 4$ once each in ascending order. The difference between the 23rd number and the 21st number is ( ).
(A) 2
(B) 18
(C) 81
(D) 99