Select the homogeneous mixture from the following:

Homogeneous mixture have uniform distribution whereas heterogeneous mixture does not have uniform composition.
a) Carbon dioxide gas dissolved in water is a homogeneous mixture.
b) Air containing suspended particles is heterogeneous mixture.
c) Soap bubbles formed by blowing air into soap solution is heterogeneous mixture.
d) Water in milk is heterogeneous mixture but it seems homogeneous. Under a microscopic examination, it is proved as heterogeneous.