So far nobody has provided hard evidence to support their claim that they have seen UFO.

问题补充：

So far nobody has provided hard evidence to support their claim that they have seen UFO.

A. an; a

B. the; the

C. 不填；a

D. 不填；an

答案：

A. an; a

B. the; the

(答案→)C. 不填；a

D. 不填；an

解析：考查冠词用法。第一空的evidence是不可数的抽象名词证据，单独使用不要冠词；第二空用a/an+单数名词表示一类事物。因为UFO是辅音发音，所以使用a。句意：到目前为止，没有人能提供有力的证据来证明他们看见过UFO。故C正确。