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The tree diagram shown illustrates the formation of two-digit numbers from the digits 1, 2, 3, and 4 without repetition. According to the diagram, how many such numbers are possible? 
A10
B12
C14
D16
Answer & Solution
Correct answer: B. 12
The figure branches from each possible tens digit to the allowable units digits, excluding repetition. Each of the 4 choices for the tens place leads to 3 choices for the units place, giving $4\times 3=12$ numbers in total.