Once upon a time，a(n) famous scientist said that life was not easy for any of us but first of all we should believe in ourselves.
A. another
B. sure
C. some
D. certain
网友回答
A. another
B. sure
C. some
(答案→)D. certain
解析：考查固定用法。A certain=some a某一个，某个（表示泛指，不确定的某一个）；句意：很久以前，一个著名的科学家说过对于我们每个人来说，生活都是不容易的，首先我们要信任我们自己。故D正确。