Земледелие появилось примерно: а) 50—40 тыс. лет тому назад; б) 12—10 тыс. лет тому назад;

1. Nick doesn't skate once a week.
gener.  Does Nick skate once a week?
или Doesn't Nick skate once a week?
subj. Who skates once a week? или Who doesn't skate once a week?
spec. How often doesn't Nick skate? 
disj. Nick doesn't skate once a week, does he?
alt. Doesn't Nick skate once or twice a week?
2. They are discussing an interesting problem
subj. Who is discussing an interesting problem?
gener. Are they discussing an interesting problem?
spec. What are they discussing?
disj. They are discussing an interesting problem, aren't they?
alt. Are they or are we discussing an interesting problem?
3. The programm started at 8 o'clock yesterday.
subj. What started at 8 o'clock yesterday?
gener. Did the programm start at 8 o'clock?
spec. When did the programm start?
disj. The programm started at 8 o'clock yesterday, didn't it?
alt. Did the programm start at 8 or at 7 o'clock yesterday?