Seven friends are living in 7 different flats of an apartment. Each of them was allotted a parking slot to park their cars in the ground floor. But they used to park their cars randomly in any of the seven slots. In how many ways can they park their cars so that exactly five persons park their cars in the slots allotted to them?  Choose the correct choice from the following answers.
A) 14	B) 21	C) 35	D) 42	E) 49
C57=21 : choosing which of the 7 persons will be on the slots allocates to them, the remaining 2 will be parked on each others slots.   Answer: 21.
<result>B</result>
Seven friends are living in 7 different flats of an apartment. Each of them was allotted a parking slot to park their cars in the ground floor. But they used to park their cars randomly in any of the seven slots. In how many ways can they park their cars so that exactly five persons park their cars in the slots allotted to them?