Nine family members: 5 grandchildren (3 brothers and 2 sisters) and their 4 grandparents are to be seated around a circular table. How many different seating arrangements are possible so that 2 sisters are seated between any two of the three brothers?
Choose the correct choice:
A) 120
B) 480
C) 1440
D) 2880
E) 8640
Consider two brothers and two sisters between them as one unit: {BSSB}.   So, now we have 6 units: {G}, {G}, {G}, {G}, {B}, and {BSSB}.   These 6 units can be arranged around a circular table in   (6-1)= 
<gadget id="calculator">  (6-1)</gadget>
<output>5</output>
5! ways.   Next, analyze {BSSB} unit:   We can choose 2 brothers out of 3 for the unit in C23=3C32=3 ways;   These brothers, within the unit, can be arranged in 2! ways: {B1,S,S,B2}{B1,S,S,B2} or {B2,S,S,B1}{B2,S,S,B1}   The sisters, within the unit, also can be arranged in 2! ways: {B,S1,S2,B}{B,S1,S2,B} or {B,S2,S1,B}{B,S2,S1,B}   Therefore, the final answer is 5!*3*2*2=1440.   Answer: 1440.
<result>C</result>
Nine family members: 5 grandchildren (3 brothers and 2 sisters) and their 4 grandparents are to be seated around a circular table. How many different seating arrangements are possible so that 2 sisters are seated between any two of the three brothers?