Grammaire anglaise : Present Perfect vs Past Simple

Complete the following sentences using either the Present Perfect or the Past Simple tense for the verbs in brackets. Explain your choice for each sentence:

1. I (visit) __________ London three times in my life.
2. She (buy) __________ a new laptop last Friday.
3. We (not / see) __________ our cousins since last summer.
4. They (finish) __________ their homework ten minutes ago.