A 3-letter code word consists of letters A, B. If the code includes all the two letters, how many such codes are possible?  Select the correct option.
A) 72
B) 48
C) 6
D) 24
E) 18
A-AB can be arranged in 3!/2!=3 ways;   B-AB can be arranged in 3!/2!=3 ways;   Total:  3+3= 
<gadget id="calculator"> 3+3</gadget>
<output>6</output>
6.   Answer: 6.
<result>C</result>
A 3-letter code word consists of letters A, B. If the code includes all the two letters, how many such codes are possible?