7. Раскройте скобки, употребляя требующуюся форму инфинитива. 1. They seemed (to

1. They seemed to be quarreling: I could hear angry voices from behind the door. To be quarreling - Continuous Infinitive. 
2. They are supposed to have been working at the problem for the last two months. To have been working - Perfect Continuous Infinitive.
3. The only sound to be heard was the snoring of grandfather in the bedroom. To be heard - Indefinite Passive Infinitive.