1.I"d like to have a glass of milk and (C)

1.I"d like to have a glass of milk and (C)
A.two breads B.two pieces of breads C.two pieces of bread
2.beijing is one of the biggest (C) in the world .
A.citys B.city C.cities
3.I met some (C) in The park and talked with them the other day.
A.japanese .B.American C.chinese
PS:问下分别选择它们的原因