Past simple vagy present perfect? Mi a különbség?

1. Would you like something to eat?

A: No thanks, I've just had lunch.

B: No thanks, I just had lunch.

2. Do you know where Julia is?

A: Yes, I've just seen her.

B: Yes, I just saw her.

3. What time is David leaving?

A: He's already left.

B: He already left.

4. What's in the newspaper today?

A: I don't know. I haven't read it yet.

B: I don't know. I didn't read it yet.

5. Is Sue coming to the cinema with us?

A: No, she's already seen the film.

B: No, she already saw the film.

6. Are your friends here yet?

A: Yes, they've just arrived.

B: Yes, they just arrived.

7. What does Tim think about your plan?

A: We haven't told him yet.

B: We didn't tell him yet.

Jelentésben mi a különbség?

Előre is köszönöm a válaszokat.