9.A, B, C, D, E, F six people are each assigned a number, and these six numbers are all different.

A says: “Our six people's numbers form an arithmetic sequence”
B says: “The smallest number in this arithmetic sequence is 2"
C says: “The sum of our six people's numbers is 42"
D says: “The sum of A, C, E's numbers is twice the sum of B, D, F's numbers”
E says: “The sum of B, F's numbers is twice A's number"
Then D's number is ( ).
【Analysis】Calculation, Logical Reasoning
According to what $\mathrm{ABC}$ said, we know that these 6 numbers are $2,4,6,8,10,12$,
According to what $\mathrm{D}$ said, we know that the sum of $A C E$ is $42 \div 3 \times 2=28, B D F$'s sum is $42 \div 3=14$; $\mathrm{BDF}$ is $2,4,8, \mathrm{ACE}$ is $6,10,12$;
Also, $\mathrm{BF}$ is $\mathrm{A}$'s 2 times, so $\mathrm{A}=6, \mathrm{~B}+\mathrm{F}$ is $4+8=12$. Therefore, $\mathrm{D}$ is 2.