worth，worthy，worth-while都為adj.　意為"值得"。
    1. worth： be worth + n.　 當名詞為金錢時，表示"…… 值得……"
    be worth doing sth.　"……某事值得被做"
    The question is not worth discussing again and again.
    2. worthy：be worthy of +n.　當名詞為抽象名詞時表示"……值得……"
    be worthy to be done "某事值得被做"
    The question is not worthy to be discussed againand again.
    3. worth-while： be worth-while to do sth　 "值得做某事"
     worth while： It is worth while doing sth
    　　　 It is worth while sb to do sth.
    典型例題
    It is not ____ to discuss the question again and again.
    A.worth　B.worthy　C.worth-while　D.worth while
    答案C. 由worth的用法可知，此句只適合詞組be worth-while to do sth.。因此選C。