The key answer of exercise 4

a)

• she's sold → Present Perfect Simple → We don’t know when this happened.
• she started → Past Simple → We do know when this happened.

b)

• We use the Past Simple to talk about an action or experience at a specific time in the past (e.g. I went to the cinema last night.)
• We use the Present Perfect Simple to talk about an action or experience at some time in the past up until now. The specific time is not important or not known (e.g. I have been to Paris twice).