The word “sandwich” now denotes a popular fast food. It originates from John Montague， Fourth Earl of Sandwich in 18th century. This example shows one of the sources of English vocabulary is（） .

The word “sandwich” now denotes a popular fast food. It originates from John Montague， Fourth Earl of Sandwich in 18th century. This example shows one of the sources of English vocabulary is（） .
A、compound
B、proper nouns
C、back-formation
D、conversion
正确答案：B