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“Last week, I took the B2 exam.” – You should use the Past Simple when speaking about a completed action which took place in a specified time in the past.
“Yesterday, I listened to a very interesting radio program.”
“When I was young, I loved sweets.”
If you are talking about the past, and the time of the discourse is already known, you don’t need to specify it:
“Last summer, we took a wonderful trip. We went to America and Asia. It took us an entire month to complete the journey.” – All three sentences express completed actions which took place last summer. So, we don’t need to point it out in each sentence.