1.  

a - had only been working

b - had finally worked

2.  

a - had carried

b - had been carrying

3.

a - had applied

b - had been applying

4.

a - had flown

b - had been flying

1. had been trying

2. had visited

3. had cost

4. had been writing

5. had been worrying

6. had arrived

7. had always believed

8. had been talking