Operation of basic exemption limits in tds?

Hey Everyone,

Please help me in getting this basic correct,

Person A gives contracts to Person B. Following events may occur

1. B charges Rs. 25000/- but its just one bill no further bills are received?
2. B charges Rs. 80000/- in a single bill (is TDS applicble in on 75000-80000= 5000 or the whole amount of 80000/-)
3. B charges Rs. 80000/- (twenty thousand in four different bills), so when will TDS provision apply? And is TDS applicable on all bills or only on the fourth bill when the Limit is actually crossed?
4. In the next year B only issues bills worth Rs. 50000/-, so will TDS be deducted on the this amount because it was being done in the previous year?