单项选择题 With legal migration, governments need to persuade voters that they are accepting immigrants who will be ------- for the country, rather than those who will ------- drain resources.（）

单项选择题 Although he was ------- in his efforts to persuade us of the justness of his cause, his past history, ...... with failures, prevented us from being any more than skeptical at his words.（）

单项选择题 While the causes of the Cretaceous-Tertiary extinction have received the most attention, Alvarez's paper points out work on the other major mass extinctions that may also be ------- for the tragedy.（）