※ 引述《freedomku (地雷王)》之銘言：
: Five people gets on an elevator on the first floor of an 11-story building.
: Assume that each person selects a destination floor from 10 possible floors
: (i.e. from 2 to 11) uniformly and independently of others.
: Also, assume that no people get on the elevator above the first floor.
: Let S be the expected number of stops this elevator makes above the first
: floor, and Z be the sum of all digits(including all decimal places) in S.
: Then which of the following statements is true
: (A) 15 <=Z<= 19
: (B) 19 <=Z<= 22
: (C) 22 <=Z<= 24
: (D) 24 <=Z<= 25
: (E) none of the above
: