1. Use the digits $1,2,3$ to form a four-digit number, requiring all three digits to appear, and the same digit should not be adjacent. How many such four-digit numbers are there?
(A) 24
(B) 18
(C) 15
(D) 12
-、1. B.
From the problem, we know that one of the digits $1, 2, 3$ must be used repeatedly, which gives us three scenarios; placing the two identical numbers in non-adjacent positions in the four-digit number also has three scenarios; placing the remaining two numbers in the remaining two positions of the four-digit number has two scenarios.
In total, there are $3 \times 3 \times 2 = 18$ different four-digit numbers.