Each Group Below Has Been Divided According to a Single

Short Answer

Each group below has been divided according to a single basis of classification. However, one item in each group does not belong; it does not fit that single basis of classification. Read each group of items carefully; then select the letter of the one item that does not belong. Next, for each group, identify the single basis of classification that includes the rest of the group.
Part A: Beds
a. double
b. twin
c. water
d. king
Part B: Basis of classification:
a. size
b. firmness
c. price