Each of the Following Groups of Things Has Been Divided

Multiple Choice

Each of the following groups of things 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, select the single basis of classification that includes the rest of the group.
-Sweaters

A) button-down
B) turtleneck
C) cotton
D) V-neck