It is clear that the core is subject to change
The correct answer to this question is letter "A. homogeneous mixture." The following characteristics describe the type of mixture, which is a homogeneous mixture. The substance is mixed uniformly throughout and each part of <span>the substance contains the same ratio of materials with the same properties</span>
The larger the piece the longer it will take to break down. This is because it has more mass that needs to be broken down.
Go here and see how:
http://www.rsc.org/learn-chemistry/resource/res00000698/using-indigestion-tablets-to-neutralise-an-acid?cmpid=CMP00005978