6. A bookstore sells the "Junior High School Math Competition" series, which includes one volume for each of the 7th, 8th, and 9th grades. Xiao Zhao, Xiao Li, and Xiao Chen all go to buy this series. Each person can buy one or none of each volume, and together they find that each volume has been purchased, and each volume has at least one person who did not buy it. How many different purchasing schemes are there? ( )
(A) 512
(B) 343
(C) 216
(D) None of the above answers is correct
6.C.

For example, for the textbooks for seventh grade, each person has 2 choices: to buy or not to buy. Therefore, 3 people have $2 \times 2 \times 2=8$ choices. However, the 2 choices where all 3 people either do not buy or all buy are not allowed, so there are actually 6 choices. Thus, there are a total of different purchasing plans $6 \times 6 \times 6=216$ kinds.