Choose the correct form

1. He's not here because he
   [has been going|has gone] to work. He'll be back at 4 o'clock.
2. The children
   [cleaned|have been cleaning] the playground yesterday.
3. She
   [has studied|has been studying] all day.
4. I
   [worked|have been working] without a break since 8 o'clock.
5. I
   [have been walking|walked] to the park last week.
6. They
   [have talked|have been talking] about it all morning.