P.5 english question help!

We should use present perfect tense here. The answer is A.
Present perfect tense is used on things that are finished but still related to the present, or things that are done and continued upto now. Simple past tense is used on events that happened at a past instance, and is commonly used with the time at which the event happened.

In this case, previously losing the ticket would deny us from getting in the concert now. So the past event is related to the present. The present perfect tense should be used.

2013-06-12 10:38:30 補充：
An instance using past tense:
We lost the tickets for the concert last night at home. Let's return to find them.

2013-06-16 11:29:01 補充：
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2013-06-19 22:08:07 補充：
I learned it in school. I recommend you to study hard to understand instead of relying on the most suitable online resources as a shortcut.

a.Bad news!We have lost the tickets for the concert.We cannot go.(lose)