A student is writing a paper and wants to know the antonym of the word "satisfactory". Which of the following resources would be most useful for the student?

Question

Quizplus · Accepted Answer

A thesaurus would provide synonyms and antonyms for a given word.
A dictionary is incorrect. A dictionary would contain the definitions of various words, but would not contain synonyms or antonyms. 
An English textbook is incorrect. An English textbook would provide background information on English grammar and usage, but would most likely not contain synonyms and antonyms for various words. 
A translator is incorrect. A translator would provide the meaning of a certain word or phrase in a different language and would not provide synonyms or antonyms.

A Student Is Writing a Paper and Wants to Know