A good doctor knows that knowledge about medicine will continue to change and that, therefore, formal professional training can never be an_____guide to good practice.

A.adaptable

B.absolute

C.vary

D.invaluable

E.obsolescent

正确答案：B

答案解析：

第二个never 填空词与前面的continue to change同义重复，never表反义所以填空填一个表示不变的词。