Ten cars are to be parked in a car park that has 20 parking spaces set out in two rows of 10. Find how many different patterns of unoccupied parking spaces are possible if:

a. The cars can be parked in any of the 20 spaces

b. The cars are parked in the same row

c. The same number of cars are parked in each row

d. Two more cars are parked in one row than in the other.